Three different levels of operator &&_Basic knowledge

The

operator can be understood at three different levels.

The first level of understanding

When the operands are both Boolean values, "&&" performs a Boolean AND operation on the two values.

Copy code The code is as follows:

x==0 && y==0 // only True is returned only when x and y are both 0

Relational operators have higher precedence than "&&".

The second level of understanding

"&&" can perform Boolean AND operation on true and false values. (False values ​​include false, null, undefined, 0, NaN and ""). Anywhere you want to use a boolean value in JS, expressions and statements will treat it as a true or false value, so in fact "&&" does not always return true and false.

Copy code The code is as follows:

null && true // =>null: left operation The number is false and returns it, the entire expression is false
true && (5 - 3) // =>2: The left operand is true, the right operand is calculated, and the result is returned

The third level of understanding

When an operator wants to return a true value or a false value, it will encounter two operation situations according to the value of the left operand: the operator first calculates the value of the left operand, if the calculation result is a false value, then the entire expression The result of the formula must also be a false value. At this time, "&&" simply returns the value of the left operand and does not calculate the right operand. If the left operand is true, "&&" evaluates the value of the right operand and returns it as the result of the entire expression.

Copy code The code is as follows:

var o = {x:1};
var p = null;
o && o.x; // =>1 o is a true value, return the value of o.x
p && p.y; // =>null: p is a false value, return it, Instead of calculating p.y

The behavior of "&&" is sometimes called "short-circuiting", and we will see that a lot of code takes advantage of this feature to conditionally execute code. For example, the following two lines of code are completely equivalent:

Copy code The code is as follows:

if(a == b) stop();
(a == b) && stop();// Equivalent to the above statement

Extension of knowledge

The operator "||", like "&&", also has some complex behaviors.

is used to select the first truth-valued expression from a set of alternative expressions:

Copy code The code is as follows:

// First check whether a is a true value, if so , return a, otherwise handle b in the same way as a
// If b is a true value, return b, otherwise return 5
var max = a || b || 5;

This usage can be used in functions to provide default values ​​for parameters:

Copy code The code is as follows:

function copy(o, p) {
p = p || {}; // If no object is passed to parameter p, use the newly created object
// ...
}

Operator precedence

For operators with the same priority, the order of operations is determined by the combination direction.

To simply remember it is:! > Arithmetic operators > Relational operators > && > || > Assignment operators