javascript core language - expressions and operators

An expression is a phrase in JavaScript, and the JavaScript interpreter will evaluate it to produce a result. Constants, variable names, array access, etc. in the program are all expressions. The most common way to combine simple expressions into complex expressions is to use operators. The original expression is the simplest expression. "Primary expression". Is the smallest unit of expression ———— No other expressions are included. Constants, literals, keywords, and variables are all primitive expressions

1.23
"hello"
/pattern/

true
false
null
this

i
sum
undefined

Initialization expressions for objects and arrays

Initialization expressions for objects and arrays are actually a newly created object and array. These expressions are sometimes called Make "object literals" and "array literals"!

[]
[1+2, 3+4]

var sparseArray = [1,,,,,5]           // 数组分割逗号之前的元素可以省略，空位默认填充 undefined
var matrix = [[1,2,3], [4,5,6], [7,8,9]]

var p = { x: 2.3, y: -1.2}
var q = {}
q.x = 2.3; q.y = -1.3

Function definition expression

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

Property access expression

// expression.identifier
// expression[expression]

var o = { x:1, y:{z:3} };
var a = [0, 4, [5, 6]];
o.x             // => 1 表达式 o 的 x 属性
o.y.z           // => 3 表达式 o.y 的属性 z
o["x"]          // => 1 对象 o 的 x 属性
a[1]            // => 4 表达式 a 中索引为 1 的元素
a[2]["1"]       // => 6 表达式 a[2] 中索引为 1 的元素
a[0].x          // => 1 表达式 a[0] 的 x 属性

/*不管使用哪种形式的属性访问表达式，在「.」和「[」 之前的表达式总是会首先计算。如果计算结果是 null 或者 undefined，表达式会抛出一个类型错误异常，因为这两个值都不能包含任意属性。如果运算结果不是对象（或者数组），JavaScript 会将其转换为对象。如果对象表达式后跟随一对方括号，则会计算方括号内的表达式的值并将它转换为字符串，不论哪种情况，如果命名的属性不存在，那么整个属性访问表达式的值就是 undefined */