Can anyone explain, why the following happens with ES6 array destructuring?
let a, b, c
[a, b] = ['A', 'B']
[b, c] = ['BB', 'C']
console.log(`a=${a} b=${b} c=${c}`)Expected: a=A b=BB c=C
Actual: a=BB b=C c=undefined
As others have said, you're missing semicolons. But…
Can anyone explain?
There are no semicolons automatically inserted between your lines to separate the "two" statements, because it is valid as a single statement. It is parsed (and evaluated) as
let a = undefined, b = undefined, c = undefined;
[a, b] = (['A', 'B']
[(b, c)] = ['BB', 'C']);
console.log(`a=${a} b=${b} c=${c}`);
wherein
[a, b] = …; is a destructuring assignment as expected(… = ['BB', 'C']) is an assignment expression assigning the array to the left hand side, and evaluating to the array['A', 'B'][…] is a property reference on an array literal(b, c) is using the comma operator, evaluating to c (which is undefined)If you want to omit semicolons and let them be automatically inserted where ever possible needed, you will need to put one at the start of every line that begins with (, [, /, +, - or `.