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Type conversions

importance: 5

What are results of these expressions?

"" + 1 + 0 "" - 1 + 0 true + false 6 / "3" "2" * "3" 4 + 5 + "px" "$" + 4 + 5 "4" - 2 "4px" - 2 " -9 " + 5 " -9 " - 5 null + 1 undefined + 1 " \t \n" - 2

Think well, write down and then compare with the answer.

"" + 1 + 0 = "10" // (1) "" - 1 + 0 = -1 // (2) true + false = 1 6 / "3" = 2 "2" * "3" = 6 4 + 5 + "px" = "9px" "$" + 4 + 5 = "$45" "4" - 2 = 2 "4px" - 2 = NaN " -9 " + 5 = " -9 5" // (3) " -9 " - 5 = -14 // (4) null + 1 = 1 // (5) undefined + 1 = NaN // (6) " \t \n" - 2 = -2 // (7)
  1. The addition with a string "" + 1 converts 1 to a string: "" + 1 = "1", and then we have "1" + 0, the same rule is applied.
  2. The subtraction - (like most math operations) only works with numbers, it converts an empty string "" to 0.
  3. The addition with a string appends the number 5 to the string.
  4. The subtraction always converts to numbers, so it makes " -9 " a number -9 (ignoring spaces around it).
  5. null becomes 0 after the numeric conversion.
  6. undefined becomes NaN after the numeric conversion.
  7. Space characters are trimmed off string start and end when a string is converted to a number. Here the whole string consists of space characters, such as \t, \n and a “regular” space between them. So, similarly to an empty string, it becomes 0.