javascript - Unable to correctly write ternary operator in JavaScript

Question

I have an undefined variable and I check it in a string concat:

var undefinedVariable = undefined;
console.log("foo" + undefinedVariable === undefined ? "bar" : undefinedVariable.toString() );

Considering that undefinedVariable is undefined, undefinedVariable.toString() is an unreachable code. However, I get this error:

Uncaught TypeError: Cannot read property 'toString' of undefined(…)

The strange thing is if I remove "foo" at the start of console.log, then the code works fine.

console.log(undefinedVariable === undefined ? "bar" : undefinedVariable.toString() );

I have tested in chrome and firefox and I get the same result so probably it is not a bug. Is there any explanation why JS engines try to run the unreachable part?

Answer 1

这是因为Operator Precedence。（+连接运算符）的优先级高于?:（三元运算符）。因此，您需要将三元条件包含在其中()，因为它与+(concatenation operator) 一起使用，而左侧不再是undefined. 利用：

console.log("foo" + (undefinedVariable === undefined ? "bar" : undefinedVariable.toString()) );

您需要告诉 JavaScript 引擎undefinedVariable单独评估，不要同时加入"foo"andundefinedVariable和评估。

var undefinedVariable = undefined;
console.log("foo" + (undefinedVariable === undefined ? "bar" : undefinedVariable.toString()) );

以上给了我：foobar。