JavaScript judgment statement

1. Comparison operations

• In js, every operation will have a return value
• >, <, ==, !=, >=, <=, ===, !==. The return value of the comparison operation is true/false, which is a Boolean value.
• During a comparison operation, if you determine whether the values ​​of two variables are equal, there ==will be an implicit conversion of the data type. js is a weakly typed language, and data types will be converted to each other.
• ===The execution efficiency is higher than that ==because the latter requires data type conversion.

console.log(a = 2);//2    赋值操作运算符，返回结果是等号右边的值。
var a = 4,
    b = 10,
    c = 4,
    d = "4";
console.log(a = "你好");//你好
console.log(a > b);//false
console.log(a == b);//false
console.log(a < b);//true
console.log(a != b);//true
console.log(a == d);//true
console.log(a === d);//false 不仅值要相等，数据类型也需要相等

2. if statement

if(条件){
    条件正确的执行代码
}else{
    否则执行的代码
}

1. In () of if, implicit type conversion will occur.
2. Only in the following six cases, the data in () in if is false. false, 空字符串, 0, null, undefined, NaN.

3. Ternary operation

1. Has a condition, a true result, and a false result.
2. Ternary arithmetic: condition? true statement: false statement. takes precedence over assignment

oBox.title = oBox.title == "a"?"b":"a";

4. switch statement

1. The judgment condition of using switch to replace if is that when multiple certain values ​​are compared, the judgment in switch is that they are all equal.
2. Add a break after each case to exit the switch loop.

switch(a){
    case 1:
        console.log(1);
        break;
    case 2:
        console.log(2);
        break;
    case 3:
        console.log(3);
        break;
    default:
        console.log("其他");
        break;
}

5. Things to note when making judgments

1. Don't use color to make judgments.
2. Do not use composite attributes to make judgments.
3. Do not use URL to make judgments, which involves string conversion and relative/absolute paths.