Here is a version that works with both readonly and mutable tuples types and a separator.

type Join<
  TElements,
  TSeparator extends string,
> = TElements extends Readonly<[infer First, ...infer Rest]>
  ? Rest extends ReadonlyArray<string>
    ? First extends string
      ? `${First}${Rest extends [] ? '' : TSeparator}${Join<Rest, TSeparator>}`
      : never
    : never
  : '';

type MutableTuple = ["a", "b", "c"];
type MutableTupleJoined = Join<MutableTuple, " | ">;
//   ^? "a | b | c"

type ReadonlyTuple = readonly ["a", "b", "c"];
type ReadonlyTupleJoined = Join<ReadonlyTuple, " | ">;
//   ^? "a | b | c"

A readonly type often comes from const assertions e.g.

const tuple = ["a", "b", "c"] as const;
type Joined = Join<typeof tuple, " | ">;

When inlining the tuple type, it is usually mutable e.g.

type Joined = Join<["a", "b", "c"], " | ">;

Probably like this:

  const result = item.map(it => ({
      name: it.name,
      syllabi: it.syllabus.map(s => s.name).join(', ')
    }));

https://jsfiddle.net/ernmtcgj/

The method array.toString() actually calls array.join() which result in a string concatenated by commas. ref

var array = ['a','b','c','d','e','f'];
document.write(array.toString()); // "a,b,c,d,e,f"

Also, you can implicitly call Array.toString() by making javascript coerce the Array to an string, like:

//will implicitly call array.toString()
str = ""+array;
str = `${array}`;

Array.prototype.join()

The join() method joins all elements of an array into a string.

Arguments:

It accepts a separator as argument, but the default is already a comma ,

str = arr.join([separator = ','])

Examples:

var array = ['A', 'B', 'C'];
var myVar1 = array.join();      // 'A,B,C'
var myVar2 = array.join(', ');  // 'A, B, C'
var myVar3 = array.join(' + '); // 'A + B + C'
var myVar4 = array.join('');    // 'ABC'

Note:

If any element of the array is undefined or null , it is treated as an empty string.

Browser support:

It is available pretty much everywhere today, since IE 5.5 (1999~2000).

References

• ECMA Specification
• Mozilla
• MSDN