JavaScript Advanced Programming (3rd Edition) Study Notes 13 ECMAScript5 New Features_Basic Knowledge

The next step should be BOM and HTML5, but since ECMAScript5 has many new changes compared to ECMAScript3, and these changes are also very interesting, I will summarize the interesting changes (not all changes) that I think in this article. But this is just a list without going into details.

1. Syntax changes

1. Keywords and reserved words

In ES3, using keywords as identifiers will result in "Identifier Expected" ” error, whereas using a reserved word for an identifier may or may not result in the same error, depending on the specific engine. In ES5, although keywords and reserved words cannot be used as identifiers, they can be used as property names of objects. Among the keywords and reserved words in ES5, let and yield are newly added. In order to ensure maximum compatibility, keywords and reserved words in ES3 or ES5 should not be used as identifiers.

2. Attribute characteristics

ES5 allows the use of user-defined attribute descriptions to override the enumerable, configurable, writable, get, set and other attributes of a given attribute. The specific method is to use static functions defined on the Object object.

3. Strict mode

The biggest grammatical change is the introduction of strict mode. You can turn on strict mode through the statement "use strict"; and add the entire script at the top of the code to enable strict mode. Adding it inside a function enables strict mode only for that function. The main changes in strict mode are:

(1) Variables must be defined before using them, that is, implicit global variables are not allowed

(2) Octal data is not used

(3) Do not use the with statement

(4) eval

A. You cannot use eval as an identifier, so you cannot name a variable or function eval

B. Variables defined in the eval() function cannot be accessed externally

(5) arguments

A. Arguments cannot be used as identifiers

B. Internal objects as functions cannot be modified arguments, so there are no longer synchronous changes between formal parameters and arguments

C. Cannot access arguments.callee

D. The arguments.caller attribute is defined in ES5, and its value is always undefined, mainly used to distinguish arguments.caller and function caller, but cannot be accessed in strict mode

(6) Function

A. Two formal parameters with the same name cannot appear in a function

B. You cannot assign a value to the caller attribute of the function

C. When calling a function without specifying an environment object, the this value will not point to the window, but to the undefined

(7) object

A. Two properties with the same name cannot appear in the object

B. When modifying an attribute whose property [[writable]] is false, an exception will be thrown instead of failing silently. The same When using delete to delete an attribute whose attribute [[configurable]] is false, an exception will also be thrown

(8) delete

You cannot use delete to delete explicitly declared variables and functions

2. Changes in built-in objects

1. Object object

(1) Inherit related methods: create(), getPrototypeOf()

(2) Attribute related methods: defineProperty(), defineProperties(), getOwnPropertyDescriptor(), getOwnPropertyNames(), keys()

(3) Anti-tampering methods: preventExtensions(), isExtensible(), seal( ), isSealed(), freeze(), isFrozen()

2. Function object

(1) Added bind() method.

(2) Normalizes the attribute caller of a function object, which is used to point to a reference to the function that calls the current function.

(3) prototype is non-enumerable

3. Array object

(1) Judgment method: Added static method Array.isArray(obj) for judgment Whether obj is an instance of an Array object.

(2) Index method: Added two methods indexOf() and lastIndexOf() for finding the index of a specified item. Use congruent (===) for matching when searching.

(3) Iteration method: added every(), some(), forEach(), map(), filter() methods.

(4) Reduce method: Added reduce() and reduceRight() methods.

4. String object

Added trim() method.

5. JSON object

Added native JSON built-in object.

6. Date object

Added Date.now(), Date.prototype.toJSON() and other methods.

7. RegExp object

In ES3, a RegExp instance is shared when using a regular expression literal, but in ES5, a new one must be created every time a regular expression literal is used. RegExp instance, just like using the RegExp constructor.