javascript - JS对象键: plus value syntax

Question

我在 d3 example 中发现了这种奇怪的语法

var heatmapChart = function(tsvFile) {
    d3.tsv(tsvFile,
    function(d) {
      return {
        day: +d.day,
        hour: +d.hour,
        value: +d.value
      };
    },

加了什么

      day: +d.day,

意思是？

Answer 1

这是一元转换。这是转换为数字的简写方法。来自 ECMAScript docs :

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

Let expr be the result of evaluating UnaryExpression.

Return ToNumber(GetValue(expr)).

var a = "45"; //typeof String
a = +a; // now typeof Number