The development of ECMAScript 6 took about four years, and after that, TC39 decided that this process was not sustainable. As a result, they moved to a yearly release cycle that would ensure new language features could make it into development sooner. The more frequent releases meant that each new edition would have a much smaller number of features than ECMAScript 6. To signify this change, future versions of the specification no longer prominently feature the edition number, and instead refer to the year in which the specification was published. As a result, ECMAScript 6 is also known as ECMAScript 2015, and ECMAScript 7 is formally known as ECMAScript 2016.
ECMAScript 7 was finalized in March 2016 and contained only two additions to the language.
The left side of an exponentiation operation cannot be a unary expression other than
--. For example, this is invalid syntax:
// syntax error let result = -5 ** 2;
-5 in this example is a syntax error because the order of operations is ambiguous. Does the
- apply just to
5 or the result of
5 ** 2? Disallowing unary expressions on the left side eliminates the ambiguity. In order to clearly specify intent, you need to include parentheses either around
-5 or around
5 ** 2, as in this example:
// ok let result1 = -(5 ** 2); // equal to -25 // also ok let result2 = (-5) ** 2; // equal to 25
The only unary operations allowed on the left side of an exponentiation operation are
--. Both of these operators have clearly-defined behavior on their operands, where a prefix
-- changes the operand before any other operations take place and the postfix versions don't apply any changes until after the entire expression has been evaluated. In both cases, they are safe to use on the left side:
let num1 = 2, num2 = 2; console.log(++num1 ** 2); // 9 console.log(num1); // 3 console.log(num2-- ** 2); // 4 console.log(num2); // 1
In this example,
num1 is incremented before the exponentiation operator is applied, so
num1 becomes 3 and the result of the operation is 9. For
num2, the value remains 2 for the exponentiation operation and then is decremented to 1.