Variable ascension#

The way a JavaScript engine works is it parses the code, gets all the declared variables, and then runs them line by line. As a result, all variable declarations will be promoted to the header of the code, which is called variable promotion.

console.log(a);
var a = 1;
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The above code first uses the console.log method to display the value of variable A on the console. The variable a has not yet been declared and assigned, so this is an error, but it does not actually report an error. Since there is a variable promotion, what really runs is the following code.

var a;
console.log(a);
a = 1;
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The final result is undefined, indicating that variable A is declared but not yet assigned.

annotation#

JavaScript provides two ways to write comments: a single-line comment that starts with //; The other is a multi-line comment placed between /* and */.

Also, because JavaScript has historically been compatible with annotations of HTML code,
are also considered legal one-line comments.

x = 1; <! -- x =2;
--> x = 3;
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In the above code, only x = 1 is executed; everything else is commented out.

Note that –> is considered a single-line comment only at the beginning of the line, otherwise it would be considered a normal operation.

function countdown(n) {
  while (n --> 0) console.log(n);
}
countdown(3)
/ / 2
/ / 1
/ / 0
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In the code above, n– > 0 is actually treated as n– > 0, so it prints 2, 1, 0.