如何在同一行中解释=和===？

Question

我遇到过这段代码

render: function() {
    var boards = [];
    for (var ii = 0; ii < this.props.numBoards; ii++) {
      // We can compare to state here so we're no longer always selecting the first board.
      var isSelected = ii === this.state.selectedIndex;
      boards.push(
        <Board index={ii} selected={isSelected}  />

      );
    }

行isSelected = ii === this.state.selectedIndex是如何执行的？

以下是完整代码：http://jsbin.com/dagavu/4/embed?css,js,output

Answer 1

以下代码分两步执行

isSelected = ii === this.state.selectedIndex

1. ii === this.state.selectedIndex // comparator operator
2. isSelected = (result of step 1) // assignment operator

Answer 2

为清楚起见，该行应写为：

isSelected = (ii === this.state.selectedIndex);

这是因为==和===用于比较，语句ii === this.state.selectedIndex将返回true或false，因此isSelected将是true或false。就像if (ii === this.state.selectedIndex)只会在比较语句为真时执行其代码块一样。