Comprehensive understanding of javascript operators and logical operators-JS Tutorial-php.cn

Comprehensive understanding of javascript operators and logical operators

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Release： 2017-04-01 09:19:06

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Logical OperatorPerforms Boolean operations on operands, often used in conjunction with relational operators . Logical operators combine multiple relational expressions to form a more complex expression. Logical operators are divided into three types: logical NOT '!', logical AND '&&', and logical OR '||'. This article will introduce these three logical operators
logical NOT
logical The non- operator is represented by an exclamation mark (!) and can be applied to any value in ECMAScript. No matter what data type the value is, this operator will return a Boolean value. The logical NOT operator first converts its operand into a Boolean value, and then inverts it
The logical NOT operator is converted into a Boolean typeConversion type and Boolean() conversionFunction is the same, except that the result is inverted at the end. And if you use two logical NOT operators at the same time, it will actually simulate the behavior of the Boolean() transformation function

console.log(!!undefined);//false
console.log(!!null);//false
console.log(!!0);//false
console.log(!!-0);//false
console.log(!!NaN);//false
console.log(!!&#39;&#39;);//false
console.log(!!false);//false

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console.log(!!{});//true
console.log(!![]);//true

console.log(!!new Boolean(false));//true
console.log(!!false);//false
console.log(!!new Boolean(null));//true
console.log(!!null);//false

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Logic Not operator is often used to control loops

//Boolean 变量(bFound)用于记录检索是否成功。找到问题中的数据项时，bFound 将被设置为true，!bFound将等于false，意味着运行将跳出while循环
var bFound = false;
var i = 0;
while (!bFound) {
 if (aValue[i] == vSearchValues) {
  bFound = true;
 } else {
  i++;
 }
}

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Logical AND
Logical AND operator consists of two An ampersand (&&) means that there are two operands. Only when both operands are true, the result will return true, otherwise it will return false

//逻辑与(&&)的真值表
第一个操作数    第二个操作数    结果
true       true        true
true       false       false
false       true        false
false       false       alse

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The logical AND operation can be applied to any type of operands, not just boolean values. If one of the operands is not a Boolean value, the logical AND operation does not necessarily return a Boolean value

The logical AND operation is a short-circuit operation. If the first operand can determine the result, then the second operand will not be evaluated. Operand evaluation

For logical AND, if the first operand is false, then no matter what the value of the second operand is, the result is false, then the first operand is returned; if If the first operand is true, then the true or false result is the same as the true or false value of the second operand, then the second operand is returned

//除了false、undefined、null、+0、-0、NaN、&#39;&#39;这7个假值，其余都是真值
console.log(&#39;t&#39; && &#39;&#39;); //因为&#39;t&#39;是真值，所以返回&#39;&#39;
console.log(&#39;t&#39; && &#39;f&#39;); //因为&#39;t&#39;是真值，所以返回&#39;f&#39;
console.log(&#39;t&#39; && 1 + 2); //因为&#39;t&#39;是真值，所以返回3
console.log(&#39;&#39; && &#39;f&#39;); //因为&#39;&#39;是假值，所以返回&#39;&#39;
console.log(&#39;&#39; && &#39;&#39;); //因为&#39;&#39;是假值，所以返回&#39;&#39;

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var i = 1;
var result = (true && i++);
console.log(result,i);//因为true是真值，所以执行i++，i是2，result是1

var i = 1;
var result = (false && i++);
console.log(result,i);//因为false是假值，所以不执行i++，i是1，result是false

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Logical AND operators can be used in multiple combinations to return the value of the first expression whose Boolean value is false

console.log(true && &#39;foo&#39; && &#39;&#39; && 4 && &#39;foo&#39; && true);// &#39;&#39;

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Relationship
Priority of operatorthan logic AND (&&) and logical OR (||) have high priority, so similar expressions can be written directly without adding parentheses

if(a+1==2 && b+2==3){
  //Todo  
}

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You can use the logical AND operator to replace the if structure

if (a == b) {
 doSomething();
}
// 等价于
(a == b) && doSomething();

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Logical AND operator is often used in

Callback functionIn use

//若没有给参数a传值，则a为默认的undefined，是假值，所以不执行a()，防止报错，如果给参数a传值，则执行函数a()
function fn(a){
  if(a){
    a();
  }
}
//等价于
function fn(a){
  a && a();
}

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Logic or
Logic The OR operator is represented by two vertical bars (||) and has two operands. Only when both operands are false, the result returns false, otherwise it returns true

//逻辑或(||)的真值表
第一个操作数    第二个操作数    结果
true       true       true
true       false       true
false       true       true
false       false       false

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Similarly, the logical OR operation can also be applied to any type of operand, not just Boolean values. If one of the operands is not a Boolean value, the logical OR operation does not necessarily return a Boolean value

The logical OR operation is also a short-circuit operation. If the first operand can determine the result, then the second operand will not be Evaluation of operands

For logical OR, if the first operand is true, then no matter what the value of the second operand is, the result is true, and the first operand is returned; If the first operand is false, then the true or false value of the result is the same as the true or false value of the second operand, then the second operand is returned

console.log(&#39;t&#39; || &#39;&#39;);//因为&#39;t&#39;是真值，所以返回"t"
console.log(&#39;t&#39; || &#39;f&#39;);//因为&#39;t&#39;是真值，所以返回"t"
console.log(&#39;&#39; || &#39;f&#39;);//因为&#39;&#39;是假值，所以返回"f"
console.log(&#39;&#39; || &#39;&#39;);//因为&#39;&#39;是假值，所以返回""

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var i = 1;
var result = (true || i++);
console.log(result,i);//因为true是真值，所以不执行i++，result是true，i是1

var i = 1;
var result = (false || i++);
console.log(result,i);//因为false是假值，所以执行i++，i是2，result是1

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Similarly, the logical OR operator can also be used in multiple combinations to return the value of the first expression whose Boolean value is true

console.log(false || 0 || &#39;&#39; || 4 || &#39;foo&#39; || true);// 4

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The logical OR operator is often used to set default values for variables

//如果没有向参数p传入任何对象，则将该参数默认设置为空对象
function fn(p){
  p = p || {};
}

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