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Explaining the result of an arithmetic expression in JavaScript
I misunderstand why the following code outputs -1 in console.
x = 42;
x = (x == 42) * -1 + (x != 42) * x;
-1
Due to Type Coercion, the comparison of x to 42 yields true and is thus translated to 1.
So 1 * -1 yields -1.
Now, (x != 42) which is false yields 0 so I have expected to get in console
-42
Because -1 + 0 * 42 MEANS -42.
So why did I get -1 in the end?
2 answers
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| user56529 | (no comment) | May 1, 2022 at 14:52 |
The concept that is important to understand here is the concept of operator precedence.
Assume you have an expression a + b * c. What does it mean? You could have the following options: (a + b) * c or a + (b * c). By adding the parentheses it can be made clear what shall be computed first, but what without the parentheses?
In mathematics as well as in all programming languages that I am aware of that allow such expressions (some languages don't use this kind of syntax at all) the meaning of a + b * c is a + (b * c). This is because the * is commonly defined to have a higher precedence than +. If of two operators one has higher precedence this means, that that one "wins" over the other when it comes to computation order. Since * has a higher precedence than the +, you first have to compute the * and afterwards the +.
Looking at the expression from the question: (x == 42) * -1 + (x != 42) * x thus means the same as ((x == 42) * -1) + ((x != 42) * x). Thus, when x has the value 42, this results in (1 * -1) + (0 * x) that is -1 + 0 which gives -1.
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Now, (x != 42) which is false yields 0
OK so far.
so I have expected to get in console "-42"
No. As you say, the expression evolves:
(x == 42) * -1 + (x != 42) * x
(1) * -1 + (0) * x
-1 + 0
Negative one plus zero is negative one.
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