Front-end interview JS— let const var

Table of contents

let const var summarizes:

var - ES5 variable declaration method

let——ES6 variable declaration method

const——ES6 variable declaration method

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let const var summarizes:

var - ES5 variable declaration method

• When the variable is not assigned a value, the variable is undefined (it is also undefined when the variable is declared using)
• Scope - the scope of var is the method scope; as long as it is defined within the method, the code after defining the variable in the entire method can be used.

let——ES6 variable declaration method

• If you use the variable directly before it is declared, an error will be reported.
• Scope - let is block scope - usually let has a smaller scope than var
• let prohibits repeated declaration of variables, otherwise an error will be reported; var can be declared repeatedly

const——ES6 variable declaration method

• const is the constant declaration method; it must be initialized when declaring a variable, and the value of the constant cannot be modified in subsequent code.
• What const actually guarantees is not that the value of the variable cannot be changed, but that the memory address pointed to by the variable cannot be changed.