Nodejs express API 500 internal server error when remove a semicolon ; - node.js

Question

I've no habit to use semicolon ; in javascript code. This is my API but this is not working. Here response showing 500 Internal Server Error
router.post('/addRows', function (req, res) {
const saveData = []
[1,2,3,4,5,6,7,8,9,10].map((i) => {
console.log(i)
saveData.push(i)
if (saveData.length === 10) {
res.status(200).json({'data': saveData});
}
})
})
but If I add semicolon ; after 2nd line then this code is working. Here response showing 200 OK
router.post('/addRows', function (req, res) {
const saveData = [];
[1,2,3,4,5,6,7,8,9,10].map((i) => {
console.log(i)
saveData.push(i)
if (saveData.length === 10) {
res.status(200).json({'data': saveData});
}
})
})
What is issue. Please describe

Answer 1

You are experiencing one of the issues with automatic semicolon insertion, the feature of JavaScript that allows semicolons to be "optional".
In your case, when you miss out the semicolon after the array assignment, the array literal on the following line is being interpreted as a reference to an element within the array, which is of course undefined. You can read your code as:
router.post('/addRows', function (req, res) {
const saveData = [][1,2,3,4,5,6,7,8,9,10].map((i) => {
console.log(i)
saveData.push(i)
if (saveData.length === 10) {
res.status(200).json({'data': saveData});
}
})
})
That may not look valid but it is - the second array literal is being parsed as a property lookup containing an expression with comma ("grouping") operators. The comma operator returns its final operand, so you can simplify the above:
router.post('/addRows', function (req, res) {
const saveData = [][10].map((i) => {
console.log(i)
saveData.push(i)
if (saveData.length === 10) {
res.status(200).json({'data': saveData});
}
})
})
When you add the semicolon you remove the ambiguity and cause the second array literal to be parsed as an array literal rather than a property accessor.

Answer 2

If the ‘;’ is missed, your code will look like:
const saveData = [][1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(...)
it is going to access element from the empty array [], this code is equals to below:
const saveData = [][10].map(...)
obviously the 10th index of an empty array is undefined, so you will get TypeError: Cannot read property 'map' of undefined
Update:
Here are posts on this topic:
Why I Prefer To Use Semicolon In JavaScript
An Open Letter to JavaScript Leaders Regarding No Semicolons