What is the difference between ecmascript5 and 6

The difference between ecmascript5 and 6: 1. There is a new block-level scope in ecmascript6, but not in ecmascript5; 2. Default function parameters can be set in ecmascript6, but not in ecmascript5; 3. In ecmascript6 Arrow functions can be used, but there are no arrow functions in ecmascript5.

The operating environment of this tutorial: windows10 system, ECMAScript 6.0&&ECMAScript 5.0 version, Dell G3 computer.

What is the difference between ecmascript5 and 6

ECMAScripts5, or ES5, is the fifth revision of ECMAScripts, which was standardized in 2009. ECMAScripts6, or ES6, is the sixth revision of ECMAScripts, which was completed in 2009. Completed in 2025, also known as ES2015ES6 is an improvement after ES5. Compared with ES5, it is more concise and improves development efficiency. Some new features of ES6:

1) let declares variables and const declares constants, two There are block-level scopes. There is no block-level scope in ES5, and var has variable promotion. In let, the variables used must be declared

2) The arrow function function definition in ES6 is no longer Use the keyword function(), but use ()=> to define

3) Template string The template string is an enhanced version of the string, marked with backtick (`), and can be used as Used as ordinary strings, it can also be used to define multi-line strings

4) Destructuring assignment ES6 allows extracting values ​​from arrays and objects and assigning values ​​to variables according to a certain pattern

5 )... The expansion operator can expand the values ​​in an array or object; it can also collect multiple values ​​into a variable

The details are as follows:

New arrow function

Simplified writing. Arrow functions are suitable when the function body has only one line; when there are multiple lines, ordinary functions can be used to increase readability. Less coding, clear structure

clarifies this. The this of traditional JS is determined at runtime, not at definition; while the this of arrow functions is determined at definition and cannot be changed, nor can it be modified by methods such as call, apply, and bind. Make it clear who this points to at run time, and there is no need to judge this point at run time. Note: The arrow function does not have its own this. Its this is the outer this, which points to the this of the previous function that is not an arrow function. Because of the mechanism of js, it points to the scope of a function that is not an arrow function.

The difference between arrow functions and ordinary functions

The declaration of ordinary functions is the highest in variable promotion, while arrow functions have no function promotion
The arrow function does not have this. The this object inside the function body is the object where it is defined rather than the object where it is used.
The arrow function does not have an arguments object. The object is in It does not exist in the function body. If you want to use it, you can use the rest parameter
The arrow function cannot be used as a constructor, cannot be new, and has no property
The call and apply methods have only parameters and no scope
The yield command cannot be used, so arrow functions cannot be used as Generator functions

Block-level scope

The let command in ES6 declares variables, and its usage is similar to var, but let adds a new block-level scope to JavaScript. There is no block-level scope in ES5. And var has the concept of variable promotion, but in let, the variables used must be declared; const declares constants

Structure assignment of variables in ES6, for example: var [a,b,c] = [0 ,1,2];

Class inheritance

ES6 no longer uses the prototype chain to implement inheritance like ES5, but introduces the concept of Class, which sounds similar to Java The syntax of object-oriented programming in is somewhat similar, but the two are different.

Extended knowledge:

The difference between var, let, and const

var can be declared repeatedly, but let cannot be declared repeatedly

var is not limited to the block level, but let is limited to the block level

var will be mapped to window (an attribute will be attached), but let will not be mapped to window

var can access variables above the declaration, while let has a temporary dead zone. Accessing the variable above the declaration will report an error

const must be assigned a value after the declaration, otherwise an error will be reported

const Define an immutable quantity, and an error will be reported if it changes

const, like let, will not be mapped to window, supports block-level scope, and an error will be reported when accessing variables above the declaration

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