Code 1:
var a = "Aaa";
var b =" Bbb";
[a, b] = [b, a];
console.log (a);
console.log (b);
result:
Bbb
Aaa
Code 2:
var a = "Aaa"
var b =" Bbb"
[a, b] = [b, a]
console.log (a);
console.log (b);
result:
Aaa
[undefined, "Aaa"]
if JS doesn"t force each line to end with a semicolon, shouldn"t the results of the above two pieces of code be the same?
Test environment: node v6.10.2
does not force a semicolon does not mean that it is correct not to write a semicolon. The interpreter will automatically add a semicolon. If you don't guarantee that you can add a semicolon exactly as you want, you may add the wrong semicolon, and the result will be wrong. Become
var a = 'Aaa';
var b = 'Bbb'[a, b] = [b, a];
console.log(a);
console.log(b);in fact, the code is the same as speaking, adding a semicolon is like adding punctuation. If you add a semicolon, the machine will go as you want. if you don't add a semicolon, the machine will follow its understanding.
give an example:
you say: I won't stay on rainy days.
that is understood by others as:
1. Stay on rainy days, but I will not stay on rainy days.
2. Stay on rainy days, but I won't stay.
3. A visitor on a rainy day, but a visitor on a rainy day? No.
4. If you are a guest on a rainy day, will you stay with me? Stay.
5. If it rains, it will be a guest. I won't stay if it rains.
6. Rainy days, guest days, stay with me? No.
7, rainy days, guest days, do you want me to stay? Stay.
8. Should I stay on rainy days or guest days?
code also has the same meaning, and the different results are just because of ambiguity. The above two answers are well explained, so I won't repeat them.
not everywhere without a semicolon. Your problem is:
var b = 'Bbb'[a, b] = [b, a];it's actually quite interesting to look back at this question. So let me continue with the analysis:
=
< H1 > explain in detail < / H1 >Why does b output at this time: [undefined, "Aaa"]?
extracurricular knowledge: first of all, there is a link
var a = b =c; where breadc is an assignment statement and an expression, which is actually var a = (breadc) , which is equivalent to:
b=c;
a=c; back to the original topic:
originally used [a code b] = [bjournal a] is an array exchange assignment with structural assignment. Once
but, becomes: var b = 'Bbb' [a, b] = [b, a] , the meaning changes to:
var b = [b, a] this is simple, obviously assigning an array to b, but the first element of the array b is undefined!
so output:
b = [undefined, "Aaa"]
it is not mandatory, but without ambiguity. Your second sentence and your third sentence are ambiguous and can be merged into one sentence.
give me another example
just look at the picture. I installed Eslint in vscode, and those who can automatically format and correct errors will automatically format and correct errors, so you can see the execution logic of the parser.
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