Include Default Parameter Values in Signature Help

From https://github.com/Microsoft/vscode/issues/28925

TypeScript Version: 2.4.0

Feature Request signatureHelp current shows when a parameter is optional but does not include any information about that parameter’s default value:

function foo(x = 10) { }
foo(|)

Result of TSServer signatureHelp on foo

[Trace  - 5:42:22 PM] Response received: signatureHelp (378). Request took 20 ms. Success: true 
Result: {
    "items": [
        {
            "isVariadic": false,
            "prefixDisplayParts": [
                {
                    "text": "foo",
                    "kind": "functionName"
                },
                {
                    "text": "(",
                    "kind": "punctuation"
                }
            ],
            "suffixDisplayParts": [
                {
                    "text": ")",
                    "kind": "punctuation"
                },
                {
                    "text": ":",
                    "kind": "punctuation"
                },
                {
                    "text": " ",
                    "kind": "space"
                },
                {
                    "text": "void",
                    "kind": "keyword"
                }
            ],
            "separatorDisplayParts": [
                {
                    "text": ",",
                    "kind": "punctuation"
                },
                {
                    "text": " ",
                    "kind": "space"
                }
            ],
            "parameters": [
                {
                    "name": "x",
                    "documentation": [],
                    "displayParts": [
                        {
                            "text": "x",
                            "kind": "parameterName"
                        },
                        {
                            "text": "?",
                            "kind": "punctuation"
                        },
                        {
                            "text": ":",
                            "kind": "punctuation"
                        },
                        {
                            "text": " ",
                            "kind": "space"
                        },
                        {
                            "text": "number",
                            "kind": "keyword"
                        }
                    ],
                    "isOptional": true
                }
            ],
            "documentation": [],
            "tags": []
        }
    ],
    "applicableSpan": {
        "start": {
            "line": 3,
            "offset": 5
        },
        "end": {
            "line": 3,
            "offset": 5
        }
    },
    "selectedItemIndex": 0,
    "argumentIndex": 0,
    "argumentCount": 0
}

When the default value is a simple literal type, it would be helpful to display this default value in the signature help. This information could be included in the displayParts response

[alystair]

Also applicable to JS, not just TS

[DavidBradbury]

I understand the desire to make this feature ‘smart’, but I think over-engineering is delaying this from getting done. In the JSDoc spec, it suggests a few examples and all you have to do is show what is literally defined in the optional parameter as text, not interpret arguments as given by the function.

Or if it is desired to show default values based on the function itself, and not JSDoc comment, this should be split into two issues instead of merging all the JSDoc related issues into this issue.