Ways to declare a variable

JavaScript has three ways to declare a variable: var, let, and const. They differ in scope, whether they can be redeclared, and hoisting behavior. Let's go through them in detail.

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#1. var - the old way (ES5 and earlier)

#Features:

• It can be redeclared and reassigned:

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  var x = 10; var x = 20; // does not cause an error

• Function scope (ignores {} blocks)

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  if (true) { var a = 5; } console.log(a); // 5 - the variable "escaped" outside the block

• Hoists - the variable is accessible before its declaration, but its value is undefined:

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  console.log(user); // undefined var user = 'Alex';

Using var in modern code is not recommended.

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#2. let - the modern way for mutable variables

#Features:

• The value can be changed, but it cannot be redeclared in the same block:

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  let a = 1; // let a = 2; Error

• Block scope:

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  if (true) { let x = 10; } console.log(x); // ReferenceError - the variable is not visible outside the block

• Not accessible before its declaration (unlike var):

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  console.log(a); // ReferenceError let a = 5;

Use let when the variable's value will change.

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#3. const - a constant (immutable reference)

#Features:

• It cannot be reassigned or redeclared:

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  const x = 10; x = 20; // TypeError

• But if it is an object or array, its internal data can be changed:

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  const user = { name: 'Tim' }; user.name = 'Alex'; // allowed // user = {} cannot reassign the reference itself

• Block scope - same as let.

Use const by default, and let only if the variable actually needs to change.

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#Quick comparison: