What is an expression in javascript

An expression statement is actually an expression, which is composed of operators connecting variables or direct quantities. Generally speaking, the expression statement is either a function call, an assignment, or an increment or decrement, otherwise the result of the expression calculation has no meaning.

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

An expression statement is actually an expression, which is composed of operators connecting variables or direct quantities.

Generally speaking, expression statements are either function calls, assignments, or self-increment or self-decrement, otherwise the result of the expression calculation has no meaning.

There is no such restriction in JavaScript syntax. Any legal expression can be used as an expression statement.

a + b;

This line of code calculates the value of the addition of a and b, but it will not be displayed or produce any execution effect (unless a and b are getters), but it does not prevent it from complying with the grammar. be executed.

PrimaryExpression Primary expression

The atomic item of expression: Primary Expression. It is the smallest unit of expression, and the grammatical structure it involves also has the highest priority.

Primary Expression contains various "direct quantities". Direct quantities are values ​​of a specific type that are written directly using a certain syntax. Direct quantities are the syntax for writing them out in code.

JavaScript can define objects in the form of direct quantities. JavaScript provides grammatical support for special object types such as functions, classes, arrays, and regular expressions.

({});
(function(){});
(class{ });
[];
/abc/g;

At the grammatical level, expression statements starting with function, { and class have grammatical conflicts with declaration statements. If you want to use such expressions, you must add parentheses to avoid grammatical conflicts.

Primary Expression can also be this or a variable. In syntax, the variable is called an "identifier reference".

this;
myVarFun;

Any expression plus parentheses is considered a Primary Expression. This mechanism makes parentheses a means of changing the priority of operations.

(a + b);

MemberExpression Member Expression

Member Expression is usually used to access object members. It has several forms:

a.b;
a["b"];
new.target;
super.b;

new.target is a newly added syntax, used to determine whether the function is called by new, super is the syntax used to access the properties of the parent class in the constructor.

Member Expression was originally designed for attribute access, but due to syntactic structure requirements, the following two types are regarded as Member Expressions in the JavaScript standard:

Template with function, this one with function The named template means that each part of the template is calculated and passed to a function.

f`a${b}c`;

The new operation with a parameter list and the new operation without a parameter list have a lower priority and do not belong to Member Expression.

new Cls();

They belong to the same priority as attribute operations, but have no semantic relationship.

NewExpression NEW expression

Member Expression plus new is New Expression (New Expression can also be formed without adding new, which is an independent high-priority expression by default in JavaScript can form low-priority expressions).

New Expression specifically refers to an expression without a parameter list. The following code:

new new Cls(1);

Intuitively, it may have two meanings:

new (new Cls(1));

new (new Cls)(1);

In fact, it is equivalent to the first one. Use the code to verify:

class Cls{
  constructor(n){
     console.log("cls", n);
        return class {
           constructor(n) {
              console.log("returned", n);
            }
        }
    }
}

new (new Cls(1));

Running results: This shows that 1 is passed in as a parameter when calling Cls.

CallExpression Function call expression

Member Expression can also constitute Call Expression. Its basic form is Member Expression followed by a parameter list in parentheses, or the super keyword can be used instead of Member Expression.

a.b(c);
super();

This seems simple, but there are some variations to it. For example:

a.b(c)(d)(e);
a.b(c)[3];
a.b(c).d;
a.b(c)`xyz`;

The forms of these variations are almost one-to-one correspondence with Member Expression. In fact, it can be understood that if a certain substructure in Member Expression has a function call, then the entire expression becomes a Call Expression. The Call Expression loses the feature of higher priority than the New Expression, which is a major difference.

LeftHandSideExpression lvalue expression

New Expression and Call Expression are collectively called LeftHandSideExpression, lvalue expression.

An lvalue expression is an expression that can be placed on the left side of the equal sign. The JavaScript syntax is:

a() = b;

Such usage is actually grammatical, but the values ​​returned by native JavaScript functions cannot be assigned. Therefore, most of the time, the assignments we see will be other forms of Call Expression, such as:

a().c = b;

According to the design of the JavaScript runtime, it is not ruled out that some hosts will provide functions that return reference types. At this time, The assignment is valid.

左值表达式最经典的用法是用于构成赋值表达式，但是其实如果翻一翻 JavaScript 标准，就会发现它出现在各种场合，凡是需要“可以被修改的变量”的位置，都能见到它的身影。

AssignmentExpression 赋值表达式

AssignmentExpression 赋值表达式也有多种形态，最基本的当然是使用等号赋值：

a = b

等号是可以嵌套的：

a = b = c = d

连续赋值，是右结合的，它等价于下面这种：

a = (b = (c = d))

先把 d 的结果赋值给 c，再把整个表达式的结果赋值给 b，再赋值给 a。

赋值表达式的使用，还可以结合一些运算符，例如：

a += b;

相当于：

a = a + b;

能有这样用的运算符有下面这几种：

*=、/=、%=、+=、-=、<<=、>>=、>>>=、&=、^=、|=、**=

赋值表达式的等号左边和右边能用的表达式类型不一样。

Expression 表达式

赋值表达式可以构成 Expression 表达式的一部分。在 JavaScript 中，表达式就是用逗号运算符连接的赋值表达式。

在 JavaScript 中，比赋值运算优先级更低的就是逗号运算符了。可以把逗号可以理解为一种小型的分号。

a = b, b = 1, null;

逗号分隔的表达式会顺次执行，就像不同的表达式语句一样。“整个表达式的结果”就是“最后一个逗号后的表达式结果”。比如之前的例子，整个“a = b, b = 1, null;”表达式的结果就是“，”后面的null。

在很多场合，都不允许使用带逗号的表达式，比如我export 后只能跟赋值表达式，意思就是表达式中不能含有逗号。

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