JS-variable types and calculations
1 knowledge points
1.1 Variable types
Value type VS reference type
Value type: Number / String / Boolean / Undefined / Null
Reference type:Object / Array / Function
Detailed explanation of typeof operator
number(NaN)、string、boolean、undefined、object（object/array/null）、function
typeof 123 //number
typeof 'abc' //string
typeof true //boolean
typeof undefined //undefined
typeof null //object
typeof { } //object
typeof [ ] //object
typeof console.log //function
(1) Undefined and null are equal; (2) NaN is not equal to any value, nor is it equal to itself.
1.2 Variable calculation-forced type conversion
String splicing
var a = 100 + 10 //110
var b = 100 + '10' //10010
== operator
100 == '100' //true
0 == ' ' //true
null == undefined //true
(1) Undefined and null are equal;
(2) NaN is not equal to any value, nor is it equal to itself.
if statement
var a = true;
if (a) {
} //a为true
var b = 100;
if (b) {
} //b为true
var c = ' ';
if (c) {
} //c为false
false, null, undefined, "", 0, NaN-judged as "fasle", the rest are "true"
logic operation
10 && 0 //0
' ' || 'abc' //'abc'
!window.abc //true
判断一个变量会被当做 true 还是 false
var a = 100
console.log(!!a)
2 Q&A
Topic:
*Types that can be obtained by using typeof in JS
*When to use === When to use ==
*What are the built-in functions in
JS*What types of JS variables are classified according to the storage method, and describe their characteristics
*How to understand JSON
2.1 Types that can be obtained using typeof in JS
number / string /boolean / undefined / object / function
(number)/(string)/(ture.false)/(undefined)/(null.object.array)/(funtion)
2.2 When to use === When to use ==
===Strictly equal, will compare the types and values of two values.
==Abstractly equal. When comparing, the type conversion will be performed first, and then the values will be compared.
– Only use == in the following cases, use === in other cases:
if (obj.a == null) {
// 这里相当于 obj.a === null || obj.a === undefined ，简写形式
// 这是 jquery 源码中推荐的写法
}
2.3 What are the built-in functions in JS
Number / String / Boolean / Object / Array / Function /
Date / RegExp / Error
2.4 What types of JS variables are classified according to the storage method, and describe their characteristics
- Distinguished into-value type, reference type;
value type: Number / String / Boolean / Undefined / Null
reference type: Object / Array / Function - Differences:
①The storage location is different.
Value type: It occupies a fixed space and is saved in the stack. The value itself is saved and copied. Typeof() can be used to detect the value type;
reference type: The occupied space is not fixed, and it is saved in the heap. The copy is a pointer to the object, you need to use instanceof() to detect the data type, and the object constructed by the new method is a reference type;
②The
direct assignment of a variable with a
different value type is a deep copy; a direct assignment of a reference type variable It is a passing reference, it is a shallow copy;
③Can you add attributes and method
value types can not, but reference types can;
2.5 How to understand JSON
JSON is a JS object
There are two methods:
①stringify: JSON.stringify converts json objects into strings
②parse: JSON.parse parses strings into json objects
JSON is also a data exchange format