Using older syntax
Prints "this is myfunc1"
myfunc1();
function myfunc1() {
console.log("this is myfunc1");
}
Using es6 syntax This gives error "myfunc2 is not a function"
myfunc2();
var myfunc2 = () => {
console.log("this is myfunc2");
}
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Ankul Singh Rajput
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Is it something related to hoisting? – Ankul Singh Rajput Jan 06 '20 at 16:17
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3This has nothing to do with ES6 syntax. You are using a variable before it is instantiated. You would have the same result doing `var myfunc1 = function() {...}` – Nicolas Jan 06 '20 at 16:20
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1@Nicolas — With `let` the error would be "Uncaught ReferenceError: Cannot access 'myfunc2' before initialization" because `var` is hoisted but `let` is not so you'd hit the temporal dead zone. – Quentin Jan 06 '20 at 16:22
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why it gives "not a function" rather than undefined? – Ankul Singh Rajput Jan 06 '20 at 16:24
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if i print a variable before it is declared, Js prints undefined. – Ankul Singh Rajput Jan 06 '20 at 16:25
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@AnkulSinghRajput — That's a standard error from trying to treat any value that isn't a function as a function. – Quentin Jan 06 '20 at 16:25
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@AnkulSinghRajput because you are using the variable rather than printing it. `console.log(myfunc1)` would give `undefined`. – Nicolas Jan 06 '20 at 16:26
1 Answers
2
Function declarations are hoisted.
Variable declarations using var keyword are also hoisted (meaning the Javascript engine "knows" the variable has been declared) but, because assignments are not hoisted, the variable will contain undefined until the line of code that does the assignment is executed.
Do not consider this "old" and "new" syntax.
Function declarations, function expressions, and arrow functions all have different behaviour. While arrow functions were introduced to the language more recently, they are not a replacement for function declarations or function expressions.
