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Variables are used to store this information. A variable.A variable is a “named storage” for data.… To create a variable in JavaScript, use the let keyword.… Variable naming.There are two limitations on variable names in JavaScript: The name must contain only… An extra variable is good, not evil.… let – is a modern variable declaration. var – is an old-school variable declaration.

We already know that a function can access variables outside of it (“outer” variables).… We’ll talk about let/const variables here In JavaScript, there are 3 ways to declare a variable… What’s going on with the variables here?… to a non-existing variable creates a new global variable, for compatibility with old code).… They analyze variable usage and if it’s obvious from the code that an outer variable is not used – it

So functions and variables, declared inside eval, are not visible outside:… Please note that its ability to access outer variables has side-effects.… So minifiers don’t do that renaming for all variables potentially visible from eval.… So it can’t see local variables.… Can access outer local variables. That’s considered bad practice.

In the very first chapter about variables, we mentioned three ways of variable declaration: let const… declaration var Variable assignment =.… But variables are undefined until the assignments.… So the code executes right away and has its own private variables.… Block-level variables is such a great thing.

We can swap more than two variables this way.… In the simplest case, that’s a list of variable names in {...}.… into the variable named w, then we can set the variable name using a colon:… Of course, we could use existing variables too, without let. But there’s a catch.… Note that there are no variables for size and items, as we take their content instead.

The global object provides variables and functions that are available anywhere.… There should be as few global variables as possible.… The code design where a function gets “input” variables and produces certain “outcome” is clearer, less… prone to errors and easier to test than if it uses outer or global variables.… The global object holds variables that should be available everywhere.

It references the Lexical Environment from where it’s created (we covered that in the chapter Variable… What if it could access the outer variables?… So if new Function had access to outer variables, it would be unable to find renamed userName.… If new Function had access to outer variables, it would have problems with minifiers.… Hence, they cannot use outer variables.

The ideal name for a variable is data. Use it everywhere you can.… After all, a variable eventually gets a value.… Name a variable by its type: str, num… Give them a try.… Sure, the variable name still means something.… In a function try to use only variables passed as parameters.

As a result we have two independent variables… A variable assigned to an object stores not the object itself, but its “address in memory” – in other… Let’s look at an example of such a variable:… We may think of an object variable, such as user, like a sheet of paper with the address of the object… When an object variable is copied, the reference is copied, but the object itself is not duplicated.

Global variables are visible from any function (unless shadowed by locals).… Most variables reside in their functions.… Here’s one more example: we have a variable from and pass it to the function.… A function may access outer variables. But it works only from inside out.… in the function, not outer variables.

In other words, a property counter and a variable let counter are two unrelated things.… Variables are not function properties and vice versa. These are just parallel worlds.… For instance, we can rewrite the counter function example from the chapter Variable scope, closure to… There’s no local sayHi, so the outer variable is used.… The outer code still has its variable sayHi or welcome.

Variables are dynamically typed.… with two exceptions: More in: Variables… operator provides a way to choose a defined value from a list of variables. The result of a ??… But we can also omit let and reuse an existing variable.… Such variables are only visible inside the function.

For example: Here we can see a variable… Here we immediately assign it to the variable, so the meaning of these code samples is the same: “create… a function and put it into the variable sayHi”.… Both examples above store a function in the sayHi variable.… We can pass it between variables and run when we want.

The dot requires the key to be a valid variable identifier.… They allow any property names and variables. But they are also more cumbersome to write.… If we omit quotes, that means a variable should contain the actual name to be tested.… Also, we could use another variable name here instead of key.… Square brackets allow taking the key from a variable, like obj[varWithKey].

At the end, all the variables share a single value.… variable.… The operators ++ and -- can be placed either before or after a variable.… When the operator goes after the variable, it is in “postfix form”: counter++.… The “prefix form” is when the operator goes before the variable: ++counter.

We can put any type in a variable.… For example, a variable can at one moment be a string and then store a number:… things, such as JavaScript, are called “dynamically typed”, meaning that there exist data types, but variables… They allow us to embed variables and expressions into a string by wrapping them in ${…}, for example:… The typeof operator allows us to see which type is stored in a variable.

While the code is paused, we can examine current variables, execute commands in the console etc.… That’s handy when we need to stop only for a certain variable value or for certain function parameters… Scope – current variables. Local shows local function variables.… Global has global variables (out of any functions).… Then we can analyze variables in the debugger to see what went wrong.

In other words, top-level variables and functions from a module are not seen in other scripts.… user.js should export the user variable. hello.js should import it from user.js module.… In other words, with modules we use import/export instead of relying on global variables.… That said, making such global variables is frowned upon. Please try to avoid them.… The resulting file is minified (spaces removed, variables replaced with shorter names, etc).

For instance, HTML has tags and attributes, JavaScript code has functions, variables, and so on.… E.g. we have a code string let varName = "value", and we need to read the variable name from… We’ll look for variable name using regexp \w+.… Actually, JavaScript variable names need a bit more complex regexp for accurate matching, but here it

The lookup of this is made exactly the same way as a regular variable search: in the outer lexical environment… Arrows have no “arguments”.Arrow functions also have no arguments variable.… Here we had to create additional variables

Let’s say we have a user’s data in variables firstName, lastName or nickName.… We’d like to display the user name using one of these variables, or show “Anonymous” if all of them are… In practice though, we may want to use default value only when the variable is null/undefined.… It’s used to assign default values to variables:

For instance: The currently executing function, its local variables and parameters.… Other functions on the current chain of nested calls, their local variables and parameters.… Global variables. (there are some other, internal ones as well) These values are called roots.… For instance, if there’s an object in a global variable, and that object has a property referencing another… The global variable "user" references the object {name: "John"} (we’ll call it John

Any expression or variable can be a loop condition, not just comparisons: the condition is evaluated… our case: Inline variable… declaration Here, the “counter” variable i is declared right in the loop.… This is called an “inline” variable declaration.… Such variables are visible only inside the loop.

Getting the first truthy value from a list of variables or expressions.… For instance, we have firstName, lastName and nickName variables, all optional (i.e. can be undefined… If all variables… becomes obvious if an operand isn’t just a value, but an expression with a side effect, such as a variable

The variable before ?. must be declared If there’s no variable user at all, then user?.… The variable… The optional chaining works only for declared variables.

Also, there’s a global variable… …That’s unless we declare a JavaScript variable… Please don’t use id-named global variables… Also, when one reads JS code and doesn’t have HTML in view, it’s not obvious where the variable comes

A comparison result can be assigned to a variable… Don’t use comparisons >= > < <= with a variable which may be null/undefined, unless you’re… If a variable can have these values, check for them separately. Summary.… Be careful when using comparisons like > or < with variables that can occasionally be null/undefined

before declarations.We can label any declaration as exported by placing export before it, be it a variable… For instance, let’s import sayHi into the local variable hi for brevity, and import sayBye as bye:… Usually, to avoid that and keep the code consistent, there’s a rule that imported variables should correspond… recalling what they mean: Before declaration of a class/function/…: export [default] class/function/variable… "module" Import the module (its code runs), but do not assign any of its exports to variables

from memory if there were no strong references to it (if we also overwrote the value of the admin variable… Now let’s take the user variable as the “referent” and create a weak reference from it to the admin variable… In our case — this is the user variable:… a weak reference using the admin variable: Then, at some point,… This way, the windowElementRef variable holds a weak reference to the DOM-element.

that contains details about the execution of a function: where the control flow is now, the current variables… pow(2, 3) call. pow(2, 3).In the beginning of the call pow(2, 3) the execution context will store variables… The variables… When we finish the subcall – it is easy to resume the previous context, because it keeps both variables… The list variable is the first object in the chain, so following next pointers from it we can reach any

Sometimes, we need to assign a variable depending on a condition.… We don’t assign a result to a variable here.

The err variable (we can use any name for it) will contain an error object with details about what happened… For instance, for an undefined variable that’s "ReferenceError". message Textual message about… Like a programming error (variable is not defined) or something else, not just this “incorrect data”… Variables are local inside try...catch...finally Please note that result and diff variables

So we can’t copy import to a variable or use call/apply with it. It’s not a function.

But just as we expose methods to interact with our component, we can expose CSS variables (custom CSS… For example, in shadow DOM we can use --user-card-field-color CSS variable to style fields, and the

Technically, it’s also possible to access the object without this, by referencing it via the outer variable… If we decide to copy user to another variable, e.g. admin = user and overwrite user with something else

We can choose to get the first parameters as variables… Here the first two arguments go into variables and the rest go into titles array:

The <template> element does not feature any iteration mechanisms, data binding or variable substitutions

In the example below, the initialization of variables, the main loop and returning the result are split… The great thing about them is that style-checking can also find some bugs, like typos in variable or

button (mousedown) – remember the distance from the pointer to the left-upper corner of the ball in variables… Resultstyle.cssindex.htmlNow we have the current “drop target”, that we’re flying over, in the variable

Developer tools allow us to see errors, run commands, examine variables, and much more.

Here hi = user.hi puts the function into the variable

But, as we can see in line (**), the value of the old div variable hasn’t changed!… outerHTML assignment does not modify the DOM element (the object referenced by, in this case, the variable… The new HTML wasn’t saved to any variable.

So in onload we can use script variables

extensible program-code-template for creating objects, providing initial values for state (member variables

But we’d like to know when it happens, to use new functions and variables from that script.

We need to write the variable name twice.… So, if we compare arrays with ==, they are never the same, unless we compare two variables that reference

In the example above the most nested callback has access to all variables script1, script2, script3.

That’s because we need the value from the variable

ultimate replacement power, because it gets all the information about the match, has access to outer variables

A function references the outer lexical environment, so, while it lives, outer variables live too.