JavaScript从零开始02 | Types

JavaScript Type: Strings

String in JavaScript are sequences of Unicode characters. 以下variable declaration在JS中是valid的：

const 你好  = 'hello';
console.log(你好); // hello

JS中没有dedicated char type, 如果需要表示char的话，一般用string来代替。String有length property：

'hello'.length; // 5

String属于Object，可以对String做类似于以下的operations：

'hello'.charAt(0); // "h"
'hello, world'.replace('world', 'mars'); // "hello, mars"
'hello'.toUpperCase(); // "HELLO"

JavaScript: Other Types

JS distinguishes between null and undefined. null specifically stands for non-value, whereas undefined stands for uninitialized value.

const a = [1, 2, 3, 4];
a[5]; // undefined
a[5] = null; 
a; // [1, 2, 3, 4, undefined, null];

JS支持Boolean Type，具体如下：

1. false, 0, empty string "", NaN, null以及undefined都会被视作false。
2. 以上之外的value都会被视作true。

Boolean(''); // false
Boolean(234); // true

Variables

JS有三种declare variable的方式，namely var, let和const。
let会declare block-level variable。

// myLetVariable is *not* visible out here

for (let myLetVariable = 0; myLetVariable < 5; myLetVariable++) {
  // myLetVariable is only visible in here
}

// myLetVariable is *not* visible out here

同样的，const也会declare block-level variable，但与let的区别在于constdeclare的variable不可以re-assign value。

const foo = 'bar';
foo = 'hello'; // will throw an error

var在ES6 standard中已经很少被使用了，因为它会automatically declares a global variable。 Traditionally, var is the only way to declare a variable in JS.

// myVarVariable *is* visible out here 

for (var myVarVariable = 0; myVarVariable < 5; myVarVariable++) { 
  // myVarVariable is visible to the whole function 
} 

// myVarVariable *is* visible out here

Note:

• If you declare a variable without assigning its value, it will be undefined initially.
• 在ES6之前，JS是没有block-level scope的，只有function存在scope，这一点和Java等语言是有区别的。然而，ES6新增的let和const允许declare block-scoped variables。

Operators

JS支持基本的operators，包括+, -, *, /以及%。

x += 5;
x = x + 5; // equivalent to above
x++; // ++ and -- can be used

+ operator will do string concatenation:

'hello' + ' world!'; // hello world!

If you add a string to a number, they will be converted into string first:

'3' + 4 + 5; // '345'
3 + 4 + '5'; // '75'

Comparisons in JS is a bit interesting. Double equal sign will perform type coersion if you give different types:

123 == '123'; // true
1 == true; // true
// To avoid coersion, use triple-equal sign
123 === '123'; // false
1 === true; // false

JS also supports bit-wise operations.

• & (bitwise AND)
• | (bitwise OR)
• ^ (bitwise XOR)
• ~ (bitwise NOT)
• << (left shift)
• >> (arithmetic right shift)
• >>> (logical right shift)
  Different from C, logical right shift can be done by the >>> operator.