What is an expression in JavaScript

In JavaScript, an expression is a combination of numbers, operators, number grouping symbols (parentheses), free variables and constraint variables, etc., in a meaningful arrangement that can obtain numerical values; the expression can be divided There are two types: primitive expressions and complex expressions.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

Generally speaking, regarding the basic syntax of JavaScript, the terms that people hear more are operators and statements. However, there is actually another term that is often used but rarely mentioned, which is JavaScript expression. This article will introduce javascript expressions in detail

#What is an expression?

An expression is a combination of numbers, operators, number grouping symbols (parentheses), free variables and constraint variables, etc., in a meaningful arrangement that can obtain numerical values. Constraint variables are assigned values ​​within the expression, while free variables can be assigned values ​​outside the expression.

Javascript expressions are divided into primitive expressions and complex expressions

Primary expression (primary expression)

##　Primary Expression is the smallest unit of expression - it no longer contains other expressions

The original expression is divided into literals, keywords and variables; in detail, it includes this keyword, identifier reference, literal References, array initialization, object initialization and grouping expressions

PrimaryExpression : 
this Identifier 
Literal 
ArrayLiteral 
ObjectLiteral 
( Expression )

this keywords and identifiers

this;//返回当前对象
i;//返回变量i的值
sum;//返回变量sum的值

Literals

Literal, also translated into direct quantity, is the data value used directly in the program

Literal ::
   NullLiteral
   BooleanLiteral
   NumericLiteral
   StringLiteral 
   RegularExpressionLiteral

null;
undefined;
true;
false;
1;
'abc';
/pattern/;

Array and object initialization

Actual array initialization and object initialization The above is an initialization process described in a literal way. These two initialization expressions are sometimes called "object literals" and "array literals"

[];
[1,2,3];
{};
{a:1};

Grouping expressions

Grouping expressions are actually parentheses, The precedence used for overriding operators

Complex expression (MemberExpression)

A complex expression consists of the original expression and the operator ( operator), including attribute access expressions, object creation expressions and function expressions

MemberExpression : 
MemberExpression [ Expression ] 
MemberExpression . IdentifierName 
new MemberExpression Arguments
FunctionExpression

Attribute access expression

The attribute access expression operation can be obtained The value of an object attribute or an array element. JavaScript defines two syntaxes for attribute access

MemberExpression . IdentifierName 
MemberExpression [ Expression ]

The first way of writing is an expression followed by a period and an identifier. The expression specifies the object, and the identifier specifies the name of the attribute that needs to be accessed

The second way of writing is to use square brackets, and inside the square brackets is another expression (this method applies to objects and arrays). The second expression specifies the name of the property to be accessed or represents the index of the array element to be accessed

var o = {x:1,y:{z:3}}; //对象字面量
var a = [o,4,[5,6]]; // 包含对象的数组字面量
o.x;//表达式o的x属性
o.y.z;//表达式o.y的z属性
o['x'];//对象o的x属性
a[1];//表达式a中索引为1的元素

Regardless of which form of property access expression is used, the expression before '.' and '['

is always evaluated first. If the evaluation result is null or undefined, the expression will throw a TypeError exception, because neither value can contain any properties.

If the evaluation result is not an object , JavaScript will convert it into an object

If the object expression is followed by a period and an identifier, the attribute value specified by this identifier will be found and returned as the value of the entire expression

　If an object expression is followed by a pair of square brackets, the expression within the square brackets will be evaluated and converted to a string

In either case, if the named property does not exist, then The value of the entire property access expression is undefined

Object creation expression

Object creation expression creates an object and calls a function to initialize the properties of the new object

new Object();
new Point(2,3);

If an object creation expression does not require passing any parameters to the constructor, then this pair of empty parentheses can be omitted

new Object;

Function expression

Function expressions are divided into function definition expressions and function call expressions

Function definition expressions define a javascript function, and the value of the expression is the newly defined function

A typical example The function definition expression contains the keyword function, followed by a pair of parentheses. Inside the brackets is a comma-separated list containing zero or more identifiers (parameter names), followed by a JavaScript code segment (function body) wrapped in brackets

function square(x){
    return x*x;
}

Function definition expressions can also contain the name of the function, and functions can also be defined through function statements instead of function expressions

var square = function(x){return x*x;}

Function A call expression is a syntactic representation of calling or executing a function or method. If this expression is a property access expression, then this call is called a method call

f(0);
Math.max(x,y,z);
a.sort();

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