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In Javascript, what does it mean when there is a logical operator in a variable declaration?

Just a quick question. When I declare a variable like this:

    var ballctx = ctx['ballctx'] || createCanvas('game-screen', 'ballctx');

Does it try the left first or the right first? I want it to create a new canvas if ctx of ballctx does not exist. If it does, it use that instead.

Answer 1

The left first. || uses what is called short-circuit evaluation, also known as minimal evaluation, meaning it will only evaluate the right side of the expression, if the left side is false.

From the ECMAScript Language Specification:

11.11. Binary Logical Operators

The production LogicalORExpression : LogicalORExpression ||
LogicalANDExpression is evaluated as follows:

1. Let lref be the result of evaluating LogicalORExpression.
2. Let lval be GetValue(lref).
3. If ToBoolean(lval) is true, return lval.
4. Let rref be the result of evaluating LogicalANDExpression.
5. Return GetValue(rref).

Thus, in your expression:

ctx['ballctx'] || createCanvas('game-screen', 'ballctx');
^-- lval          ^-- rval

If lval evaluates to true, rval won’t be evaluated. In other words, you’ll only create a new canvas if ctx['ballctx'] evaluates to false, so your code is correct.