Does Function Expression Get Hoisted in JavaScript?

In JavaScript, a function expression (defined using let, const or var) is not hoisted. This means that you cannot use a function expression before it's evaluated (i.e. until the relevant line of code is executed). If you try to access it before, you will get an error.

Function Expressions Defined Using let or const

When you use let or const to define a function expression, the following is true:

• The variable declaration/name does get hoisted, however, it is uninitialized;
• The variable assignment (i.e. function definition in this case) is not hoisted, and is inaccessible till the relevant line of code is executed/evaluated;
• Accessing the variable before the relevant line of code is executed/evaluated, results in a ReferenceError (because the variable is in a "temporal dead zone").

For example:

// ReferenceError: notHoisted is not defined
notHoisted();

const notHoisted = function() {
    console.log('foo');
};

// ReferenceError: notHoisted is not defined
notHoisted();

let notHoisted = function() {
    console.log('foo');
};

Function Expressions Defined Using var

When you use var to define a function expression, the following is true:

• The variable declaration/name does get hoisted, however, it is initialized to undefined;
• The variable assignment (i.e. function definition in this case) is not hoisted, which is why in lines before it is defined, it equals to undefined (as that's the value the variable is initialized to);
• Since undefined is not a function, a TypeError is thrown if you try to call the function anywhere before the line where it is defined.

For example:

// TypeError: notHoisted is not a function
notHoisted();

var notHoisted = function() {
    console.log('foo');
};