JavaScript hoisting refers to the behavior of moving variable and function declarations to the top of their containing scope during the compilation phase of the code. This means that you can refer to variables and functions before they are actually declared in the code.
In JavaScript, there are two types of hoisting: hoisting of variable declarations and hoisting of function declarations.
Hoisting of variable declarations:
When variables are declared using the var keyword, their declarations are hoisted to the top of their containing scope. This means that you can use the variable before it is declared in the code. However, the initialization of the variable (assigning a value to it) is not hoisted, so if you try to access the variable before it is initialized, you will get the value undefined.
For example:
console.log(x); // Output: undefined
var x = 10;
In the above code, the variable x is hoisted to the top of the scope, so the console.log statement doesn't throw an error. However, since the initialization of x is not hoisted, the value is undefined.
Hoisting of function declarations:
Function declarations in JavaScript are also hoisted to the top of their containing scope. This means that you can call the function before it is actually declared in the code.
For example:
hello(); // Output: "Hello, World!"
function hello() {
console.log("Hello, World!");
}
In the above code, the function hello is hoisted to the top of the scope, so the hello() function call doesn't throw an error.
It's important to note that hoisting only moves the declarations of variables and functions to the top of their containing scope during the compilation phase. The actual assignments and function definitions still occur at their original places in the code.
Hoisting does not work the same way for variables declared with the let or const keywords. These variables are hoisted to the top of their containing block, but they are not initialized, so trying to access them before their declaration will result in a ReferenceError.