What is the use of = sign (single) in a condition?

Question

I had read in some articles that in some languages, like in JavaScript, assignment operators can be used in conditional statements. I want to know what is the logic behind that operation? As far as I know, only comparison operators are allowed in condition checking statements.

Answer 1

Any expression is allowed in a condition checking statement. If the value of the expression isn't boolean, then it will be converted to boolean to determine what the statement should do.

You can for example use a number in an if statement:

if (1) { ... }

Any non-zero number will be converted to true.

In Javascript an assignment is also an expression, i.e. it has a value. The value of the expression is the same value that was assigned to the variable.

So, you can use an assignment expression in a condition checking statement, and the value of the expression is converted to boolean if needed:

if (x = 1) { ... }

Using an assignment in an condition checking statement can be useful, if the value that you assign should be used to control what happens. If you for example have a function that returns different values for the first calls, then a null when there are no more values, you can use that in a loop:

while (line = getLine()) {
  document.write(line);
}

You can of couse do that with the assignment separated from the logic, but then the code gets more complicated:

while (true) {
  line = getLine();
  if (line == null) break;
  document.write(line);
}