Force conversion method: 1. Use ToString(), ToNumber() or ToBoolean() method; 2. Use String(), Number() or Boolean() method; 3. Use " " or "- "operator; 4. Use statements such as "if()" and "while()".

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

1. Type conversion rules

1.ToString

For ordinary objects, Unless you define the toString method yourself, the Object.prototype.toString() method will be called. If the object has its own toString method, stringification will call the method and use its return value.

2.ToNumber

• For basic types, true is converted to 1 and false is converted to 0; undefined is converted to NaN, null is converted to 0.
• The object type will first be converted into a basic type value. If a non-numeric basic type value is returned, it will then be forced to be converted into a number according to the basic type conversion rules. In order to convert a value to the corresponding primitive type value, The abstract operation ToPrimitive first checks whether the value has a valueOf() method. If there is and returns a basic type value, then use this value for cast. If not, use the return value of toString() (if it exists) to perform the cast. If neither valueOf() nor toString() returns a basic type value, a TypeError will be generated.

let a = {
    valueOf:function(){
        return "42";
    }
};
let b = {
    toString:function(){
        return "42";
    }
};
let c = [4,2];
c.toString = function(){
    return this.join(""); //"42"
};

Number(a);     //42
Number(b);     //42
Number(c);     //42
Number('');    //0
Number([]);    //0
Number(["abc"];//NaN

3.ToBoolean

Values ??in JavaScript can be divided into values ??that can be forced to false and values ??that can be forced Converts two types of values ??to true.
False value:

• undefined
• null
• false
• 0, -0, NaN
• ""

True value: Except for the above list, all objects can be understood as true values, and all objects are true values

let a1 = new Boolean( false );
let b1 = new Number( 0 );
let c1 = new String( "" );
let d1 = Boolean(a1 && b1 && c1); //true

let a2 = [];
let b2 = {};
let c2 = function(){};
let d2 = Boolean(a2 && b2 && c2); //true

2. Display forced type conversion

1. Display conversion between strings and numbers

Strings and numbers The conversion is through the two built-in functions String(..) and Number(..).
In addition to String(..) and Number(..), there are other methods to achieve explicit conversion between strings and numbers:

let a = 42;
let b = a.toString();
let c = "3.14";
let d = +c;
b; // "42"
d; // 3.14

a.toString() is explicit , but there is an implicit conversion involved. Because toString() doesn't work with primitive values ??like 42, the JavaScript engine automatically creates a wrapper object for 42 and then calls toString() on that object. This explicit conversion contains implicit conversion.

2. Display conversion to Boolean value

let a = "0";
let b = [];
let c = {};
let d = "";
let e = 0;
let f = null;
let g;
Boolean( a ); // true
Boolean( b ); // true
Boolean( c ); // true
Boolean( d ); // false
Boolean( e ); // false
Boolean( f ); // false
Boolean( g ); // false

Although Boolean(..) is explicit, it is not commonly used . The most common way to explicitly cast a type to a Boolean value is!!

let a = "0";
let b = [];
let c = {};
let d = "";
let e = 0;
let f = null;
let g;
!!a; // true
!!b; // true
!!c; // true
!!d; // false
!!e; // false
!!f; // false
!!g; // false

[Recommended learning: javascript advanced tutorial]

3. Implicit cast conversion

1. Implicit cast conversion between strings and numbers

( 1) The operator

can be used for both numerical addition and string concatenation

if one of the operands is a string (or If a string is obtained by calling the ToPrimitive abstract operation on it), string concatenation is performed; otherwise, numeric addition is performed. Because the array's valueOf() operation cannot get a simple primitive value, it calls toString() instead. So the two arrays in the example below become "1,2" and "3,4". Concatenating them returns "1,23,4".

let a = "42";
let b = "0";
a + b; // "420"

let c = 42;
let d = 0;
c + d; // 42

let x = [1,2];
let y = [3,4];
x + y; // "1,23,4"

You can use operators for string conversion, compare numbers with the empty string "" to convert them into strings

let a = 42;
let b = a + "";
b; // "42"

a ""(implicit) and the preceding String There is a subtle difference to note between (a)(explicit).

According to the ToPrimitive abstract operation rules, a "" will call the valueOf() method on a, and then convert the return value into a string through the ToString abstract operation. String(a) calls ToString() directly. They finally return strings, but if a is an object rather than a number, the result may be different

let a = {
    valueOf: function() { return 42; },
    toString: function() { return 4; }
};
a + "";         // "42"
String( a );    // "4"

(2) Case of cast from string to number

-Yes Numeric subtraction operator, so a - 0 will cast a to a number. You can also use a * 1 and a /1, because these two operators only apply to numbers, but such usage is less common

let a = [3];
let b = [1];
a - b; // 2

2. Implicit mandatory type Convert to Boolean value

Implicit cast conversion of Boolean value will occur in the following situations:

(1) Conditional judgment expression in if (..) statement .

(2) The conditional expression (second) in the for ( .. ; .. ; .. ) statement.

(3) Conditional expressions in while (..) and do..while(..) loops.

(4) ? :Conditional judgment expression in .

(5) The operand on the left side of logical operators || (logical OR) and && (logical AND) (as a conditional expression).

In the above case, non-Boolean values ??will be implicitly cast to Boolean values, following the ToBoolean abstract operation rules introduced earlier.

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