Can't understand the conditional execution flow [duplicate]

Answer 1

It's because != does implicit type conversion. If you used the strict version, !==, it would do what you expect. But the loose version, !=, will convert both of those operands to numbers, and both "" and false convert to 0, so "" != false is false, because it ends up (through a series of convolutions) being 0 != 0.

This is laid out in detail in Abstract Equality Comparison algorithm in the specification:

1. ReturnIfAbrupt(x).
2. ReturnIfAbrupt(y).
3. If Type(x) is the same as Type(y), then Return the result of performing Strict Equality Comparison x === y.
4. If x is null and y is undefined, return true.
5. If x is undefined and y is null, return true.
6. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
7. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
8. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
9. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
10. If Type(x) is either String, Number, or Symbol and Type(y) is Object, then return the result of the comparison x == ToPrimitive(y).
11. If Type(x) is Object and Type(y) is either String, Number, or Symbol, then return the result of the comparison ToPrimitive(x) == y.
12. Return false.

As we can see from the above, if we start out with false and "", then:

• We follow Step 8, convert false to 0, and start again with 0 != ""
• We follow Step 6, convert "" to 0, and start again with 0 != 0
• We follow Step 3 and get the result false (because we're doing !=, whereas the algorithm is defined in terms of ==).