JavaScript hoisting is basically that declarations are moved to the top of your code.
Actually your function and variable declarations are added to memory during the compile phase.
And they stay exactly where you typed them in your code.
JavaScript hoists all declarations
JavaScript hoists all declarations. Initializations are not hoisted.
Declare and initialize a variable, e.g. var x = 8;, only the var x; part (the declaration) is hoisted.
The x = 8; (the initialization) is not hoisted.
A variable that is declared but not initialized is automatically set as undefined.
Take a look at the examples to understand this.
Hoisting Variables Examples
// This is how you would expect to write the code var x = 8; // Initialize and assign 8 to x console.log('X has the value: '+ x); // The printed text is: "X has the value: 8" // Here y is assigned before it is declared y = 8; // Assign 8 to y console.log('Y has the value: '+ y); // The printed text is: "Y has the value: 8" var y; // Declare y
Hoisting Variables Undefined Examples
// Declare and assign the variable at the end // Only the declaration is hoisted console.log('Z has the value: '+ z); // The printed text is: "Z has the value: undefined" var z = 8; // Declare and assign 8 to z
Hoisting Functions Examples
// This is how you would expect to write the code function printName(name) { console.log("My name is " + name); } printName("Donald"); // The printed text is: "My name is Donald" // Here the function is called before it is declared printCity("London"); // The printed text is: "My favorite city is London" function printCity(name) { console.log("My favorite city is " + name); }
Let and Const Statement
Variables and constants declared with let or const are not hoisted!
Hoisting Best Practices
Hoisting is not well known to many developers.
It is considered a best practice to declare variables at the top of their respective scopes. Then you would limit undesirable effects.
Always initialize variables when they are declared. This will provide better code and avoid undefined variables.
JavaScript strict mode gives an error if variables is used before they are declared.
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