Question

我试图通过switch语句确定对象属性值是否为“truthy”。

使用此样本块：

var test = {
  foo: "bar"
}

switch(true) {
  case test.foo:
    console.log("success in switch");
    break
  default:
    console.log("no success in switch");
    break
}

if (test.foo) {
  console.log("success in if");
} else {
  console.log("no success in if");
}

结束记录：

"no success in switch"
"success in if"

这样做的正确方法是什么？

Answer 1

你可以这样做：

case !!test.foo:

这会强制转换为布尔值。

Answer 2

除了使用!!来强制布尔值之外，您还可以使用switch语句来评估truthy / falsy：

switch (true) {             // use a boolean to force case statement to evaluate conditionals
case (val ? true : false):  // force a truthy/falsy evaluation of val using parentheses and the ternary operator
    console.log(val + ' evaluates as truthy in the switch statement.');
    break;
default:
    console.log(val + ' evaluates as falsy in the switch statement.');
    break;
}

以下是一组功能和测试，以便您自己查看：

(function () {
    'use strict';
    var truthitizeSwitch = function (val) {
            switch (true) {             // use a boolean to force case statement to evaluate conditionals
            case (val ? true : false):  // force a truthy/falsy evaluation of val using parentheses and the ternary operator
                console.log(val + ' evaluates as truthy in the switch statement.');
                break;
            default:
                console.log(val + ' evaluates as falsy in the switch statement.');
                break;
            }
            return !!val;   // use !! to return a coerced boolean
        },
        truthitizeIf = function (val) {
            if (val) {      // if statement naturally forces a truthy/falsy evaluation
                console.log(val + ' evaluates as truthy in the if statement.');
            } else {
                console.log(val + ' evaluates as falsy in the if statement.');
            }
            return !!val;   // use !! to return a coerced boolean
        },
        tests = [
            undefined,                              // falsey: undefined
            null,                                   // falsey: null
            parseInt('NaNificate this string'),     // falsey: NaN
            '',                                     // falsey: empty string
            0,                                      // falsey: zero
            false,                                  // falsey: boolean false
            {},                                     // truthy: empty object
            {"foo": "bar"},                         // truthy: non-empty object
            -1,                                     // truthy: negative non-zero number
            'asdf',                                 // truthy: non-empty string
            1,                                      // truthy: positive non-zero number
            true                                    // truthy: boolean true
        ],
        i;
    for (i = 0; i < tests.length; i += 1) {
        truthitizeSwitch(tests[i]);
        truthitizeIf(tests[i]);
    }
}());

当然:)，强制性的jsFiddle：http://jsfiddle.net/AE8MU/

在switch语句中评估Truthy

2 个答案: