Remove dashes and extract string using javascript

You can use regular expression, it would be the perfect tool for this use case.

try:

let str="6850-01-347-09";
let parts = str.replace(/(\d+)-(\d+)/,"$1 $2").replace(/[-]/g,"").split(" ");
alert( parts[0] );//6850
alert( parts[1] );//0134709

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Let's look at the string like this.
You have parts separated by '-'
You are interested in the first part and then whatever comes after that.
We can do this using a combination of split and join. We split the string into its parts (separated by '-').
The first part is the 6850 bit you want separately - but it can be any length or combination of characters (given the rule we are separating on '-') - and the rest of the string is all parts except the first one.
With that in mind we can do

const str = "6850-01-347-0964";

// Split them
const parts = str.split('-');

// Get the first part
console.log('First part : ' , parts[0]);

// Get the rest
console.log('Rest : ', parts.slice(1).join('-'));

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Be advised the accepted solution will not work if the first part of the string is not exactly 4 characters long
If your input is

const str = "685-01-347-0964";

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The output will be

Part 1: 685-
Part 2: 013470964 

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The above solution will produce this output for the same input

Part 1:  685
Part 2: 01-347-0964 

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To remove the '-' you can amend like this (just remove the '-' from the join)

const str = "685-01-347-0964";

// Split them
const parts = str.split('-');

// Get the first part
console.log('First part : ' , parts[0]);

// Get the rest
// to Join without the '-' just remove it from the join call
console.log('Rest : ', parts.slice(1).join(''));

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Which will produce the output

First part :  685
Rest :  013470964

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If your input will always be 4 numbers followed by a '-' then you are good to go - however if the first part varies in length then your accepted solution will fail.

Good suggestion Julian, the first part is always going to be 4 numbers.