var array2 = [3,6,7,8,1];
array2.slice(1);

will produce [6, 7, 8, 1]

Note that slice copies the array's contents only one level deep. What that means technically is that nested arrays or objects within the output of slice are still references to the same arrays or objects contained in the original parent array. What that means to you practically is that changing an element or property in the nested arrays/objects returned from slice will also change the same element or property within the nested array or object contained in the original parent array.

People here are making this waaay too difficult. Just do the following...

myArray.findIndex(element => element.includes("substring"))

findIndex() is an ES6 higher order method that iterates through the elements of an array and returns the index of the first element that matches some criteria (provided as a function). In this case I used ES6 syntax to declare the higher order function. element is the parameter of the function (which could be any name) and the fat arrow declares what follows as an anonymous function (which does not need to be wrapped in curly braces unless it takes up more than one line).

Within findIndex() I used the very simple includes() method to check if the current element includes the substring that you want.

var array= ["abcdefg", "hijklmnop"];
var newArray = array.filter(function(val) {
    return val.indexOf("mnop") !== -1
})
console.log(newArray)

You could map the part with a regular expression for the digits and return a number.

const
    result = [{ ID: 1, Reference: 'Id: 123, Name: "first' }, { ID: 2, Reference: 'Name: "second' }, { ID: 3, Reference: 'Id: 133, Name: "third' }],
    filter = result
        .filter(i => i.Reference.includes('Id:'))
        .map(i => i.Reference)
        .map(s => +s.match(/Id:\s*(\d+)/)[1])

console.log(filter);