JS Basic knowledge: expressions

First, what is an expression?

I understand the expression: when a program executes to 1 expressions, it returns 1 values to the location of the expression.

var a = ten, B = +;  // This is the initialization variable Console.log (a//  ) calculates the value of the variable A, B (identifier resolution), and then returns the value of both to its location. Then it is equivalent to Console.log (10+20)

Ii. several common expressions

(i) original expression

1. Direct volume (literal)

// Digital // string

Reserved words for 2.JavaScript

"Reserved words" are identifiers that have special meanings in the program, and programmers cannot use them as variables.

true // Boolean value  This // call context for a function

3. Variables

var // calculates the value of variable A, returns a 10, and then passes 10 into the alert function. 

(b) An array expression, an object expression

An array expression is a newly created array.

// creates an array of 1. 

An object expression, like an array expression, is a new 1 object.

// Create 1 Objects

(c) Function expression

Create 1 functions, either in the form of a function declaration or as a function expression. A function expression is to create 1 functions and save its references in 1 variables.

var function return // Create 1 Anonymous functions whose references are stored in the variable foo. 

(iv) Property access expressions

There are 2 types of syntax for property-access expressions:

1. "Expression. Identifier": "Expression" requires 1 objects, and "identifier" must be an attribute that already exists for the object.

var // obj.x returns the X attribute in the Obj object, 1. 

2. Expression [expression]: The 1th expression also needs to be 1 objects, and the value of the 2nd expression can be:

(1) String: Then this is the object's property access or property setting.

Property access to the object:

var obj = {x:1, y:2};  // accesses the value of the X property in the Obj object, returning 1. 

object's property setting (assigning values to 1 properties):

var // {x=1,  y=2,  z=3}

(2) Number: array element access.

var // accesses an element with an index of 0 in the array and returns 1. 

(v) Function call expression

The function call expression is this: "Function name (argument)". The return value of the function call expression, depending on the return statement.

1 did not write return statement: function call returned undefined.

    function Add (x) {            x+x;     }    Console.log (the Add (///  function does not have a return statement, the function call returns undefined.) 

2. The return statement was written, but the returned value was not written: The function call returned undefined.

    function Add (x) {            x+x;              return ;    }    Console.log (Add (// ) writes the return statement but does not return a value, the function call returns undefined. 

3. Write the return statement and the expression to return: Returns the value of the expression.

    function Add (x) {        return x+x;     }    Console.log (Add (//  ) writes the return statement, specifying the returned value (x+x), so the function call returns 2. 

Resources:

JavaScript authoritative Guide (6th edition) 4.1-4.6

JS Basic knowledge: expressions