Why does this code not result in a ReferenceError?

if(true) {
  tmp = 'abc';
  console.log(tmp);//which should throw referenceError but not

  let tmp;
  console.log(tmp);

  tmp = 123;
  console.log(tmp);
}

This code results in

abc
undefined
123

Why does the first console.log(tmp) not throw an error?

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why it should throw a referenceError

In ECMAScript 2015, let will hoist the variable to the top of the block. However, referencing the variable in the block before the variable declaration results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the declaration is processed.

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the problem is bable settings,i think.

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so,maybe it is a bug of babel? https://github.com/babel/babel.github.io/issues/826

You are correct, in ES6 this does throw an exception. There's two reasons why it doesn't for you:

• node.js already implemented let - but it works correctly only in strict mode. You should use it.
• babel does not appear to transpile the TDZ by default, as it is quite complicated and leads to lengthy code. You can however enable it with the es6.blockScopingTDZ/es6.spec.blockScoping option (but I'm not sure whether this worked in Babel 5 only and what happened in Babel 6 to them).