Quick Answer: Is Const Immutable JavaScript?

What is == and === in JavaScript?

== in JavaScript is used for comparing two variables, but it ignores the datatype of variable.

=== is used for comparing two variables, but this operator also checks datatype and compares two values.

It returns true only if both values and data types are the same for the two variables..

What is hoisting in JavaScript?

Hoisting is JavaScript’s default behavior of moving all declarations to the top of the current scope (to the top of the current script or the current function).

Is let faster than VAR?

After testing this in Chrome and Firefox, this shows that let is faster than var , but only when inside a different scope than the main scope of a function. In the main scope, var and let are roughly identical in performance. In IE11 and MS Edge, let and var are roughly equal in performance in both cases.

Is var a bad practice?

var speeds up the writing, but may slow down the reading a bit. It’s obviously not a code behaviour rule like “Always initialize variables” because the two alternatives (writing var and writing the type) have exactly the same behaviour. So it’s not a critical rule.

Can Const be changed JavaScript?

The property of a const object can be change but it cannot be change to reference to the new object. The values inside the const array can be change, it can add new items to const arrays but it cannot reference to a new array.

Is Const global in JavaScript?

Everything is global unless declared with the var keyword. There are no constants either. You can simply declare them without the var keyword.

Why is let better than VAR?

The main difference is the scope difference, while let can be only available inside the scope it’s declared, like in for loop, var can be accessed outside the loop for example. … let allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used.

When should I use const?

4 AnswersAlways use const for function parameters passed by reference where the function does not modify (or free) the data pointed to. … Always use const for constants that might otherwise be defined using a #define or an enum. … Never use const in a function prototype for a parameter passed by value.More items…•Jul 11, 2013

What is the difference between VAR and Const in JavaScript?

var declarations are globally scoped or function scoped while let and const are block scoped. var variables can be updated and re-declared within its scope; let variables can be updated but not re-declared; const variables can neither be updated nor re-declared.

Should I use const in JavaScript?

const should be used when you have a defined constant (read as: it won’t change during your program execution). If you think that it is something that may be changed on later execution then use a var .

Should I use VAR or let JavaScript?

Use let as a general rule, and var on occasion. Block scoping is the standard and most readable choice, and will make debugging easier. … Function scoping makes things a lot less apparent, and much easier to accidentally introduce bugs with scoping mistakes. In general, the smaller the scope you can use, the better.

What is use strict in JavaScript?

The “use strict” Directive It is not a statement, but a literal expression, ignored by earlier versions of JavaScript. The purpose of “use strict” is to indicate that the code should be executed in “strict mode”. With strict mode, you can not, for example, use undeclared variables.

Can I push to a const array?

Pushing to an array is considered “bad form” in functional programming. However the const declaration has nothing to do with this.

Is Let hoisted?

All declarations (function, var, let, const and class) are hoisted in JavaScript, while the var declarations are initialized with undefined , but let and const declarations remain uninitialized. They will only get initialized when their lexical binding (assignment) is evaluated during runtime by the JavaScript engine.

Why you should not use VAR?

In Javascript, it doesn’t matter how many times you use the keyword “var”. If it’s the same name in the same function, you are pointing to the same variable. … They both work with block scope, which means, if variables or constants are declared inside a block, they will not be available to the “parent” blocks.

What does Const do in JavaScript?

JavaScript const and Objects The const keyword ensures that the variable it creates is read-only. Even though the person variable is a constant, you can change the value of its property. Note that Object.

Should I use const or let?

const only prevents re-assigning, it doesn’t make the entire object immutable. … It’s useful to use const instead of let , because it prevents you from accidentally overwriting variables. So a good rule of thumb is: Stop using var .

Is Const better than let?

Turns out, const is almost exactly the same as let . However, the only difference is that once you’ve assigned a value to a variable using const , you can’t reassign it to a new value. … The take away above is that variables declared with let can be re-assigned, but variables declared with const can’t be.