Proposal for merging array or object options in tsconfig file

🔍 Search Terms

"extends", "merge", "compilerOptions", "paths", "outdir", "include", "exclude", "typeRoots", "tsconfig", "ability to merge tsconfig"

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Subset of #56436

Today when you extend a config the options written in the final config override the options from the base config. Here is sample of what happens today:

// @fileName: /temp/test/base.tsconfig.json
{
    "compilerOptions": {
        "paths": {
            "@libs/*": ["libs/*"],
            "@project1/*": ["project1/*"]
        },
        "typeRoots": ["node_modules/@types"],
    }
}

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "compilerOptions": {
        "paths": {
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["typings"]
    }
}

Here is what it would mean that project1 tsconfig was written as:

// @fileName: /temp/test/project1/computed.tsconfig.json
{
    "compilerOptions": {
        "paths": {
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["./typings"],
    }
}

Over years users have asked us for ability to create a final config that adds to base config instead of overriding. This is extremely useful for paths from compiler options.

This would allow to users to write a base config that contains common options that shouldn't have to be copied in all projects and yet allowing projects to specify there additional specific options.

For example in above sample, one would expect a way to write config such that final paths property is:

// @fileName: /temp/test/project1/computed.tsconfig.json
{
    "compilerOptions": {
        "paths": {
            "@libs/*": ["../libs/*"],
            "@project1/*": ["../project1/*"],
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["../node_modules/@types", "./typings"],
    }
}

Here are different proposals that are being considered:

Per Property Specific options

1. merge fields

Add merge as root level config property that specifies options that need to merged instead of completely overwritten in the final config.
From the above example writing the project1 config as:

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "compilerOptions": {
        "paths": {
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["typings"]
    },
    "merge": ["paths", "typeRoots"]
}

2. Inline merge options:

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "compilerOptions": {
        "merge": {
            // All the options that need to be merged
            "paths": {
               "@environment/*": ["env/*"]
            },
            "typeRoots": ["typings"]
        },
    },
}

3. merge:optionName in instead of just optionsName

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "compilerOptions": {
        "merge:paths": {
            "@environment/*": ["env/*"]
        },
        "merge:typeRoots": ["typings"]
    },
}

4. Root level merge:OptionsBagName to merge.

From the above example writing the project1 config as:

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "merge:compilerOptions": {
        "paths": {
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["typings"]
    },
}

5. merge-optionsName: true in the options

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "compilerOptions": {
        "merge:paths": true,
        "merge:typeRoots": true,
        "paths": {
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["typings"]
    },
}

6. Simple placeholder like ${base}: true or <merge>: true

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "compilerOptions": {
        "paths": {
            "${base}": true,
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["${base}", "typings"]
    },
}

7. Detailed template substitution

This should allow you to control the order esp when extending multiple configs
From the above example writing the project1 config as:

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "compilerOptions": {
        "paths": {
            "${0}": true,
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["typings", "${0}"]
    },
}

Global merge options

8. Add root level merge: true that merges all arrays and objects options:

From the above example writing the project1 config as:

// @fileName: /temp/test/project1/tsconfig.json
{
    "extends": "../base.tsconfig.json",
    "merge": true,
    "compilerOptions": {
        "paths": {
            "@environment/*": ["env/*"]
        },
        "typeRoots": ["typings"]
    },
}

Here is a chart of all options that are list or object type:

Option Category	Option	Type	Order Dependency	Eligible for Merge
CompilerOptions	paths	object	mayBe not		#44589, #20110 , For merging paths, we set pathsBasePath for resolving relative path, what happens to that when paths are merged
CompilerOptions	lib	list	no		#20110
CompilerOptions	rootDirs	list	yes?
CompilerOptions	typeRoots	list	yes		Now that we resolve types only from typeRoots, this seems to be important?
CompilerOptions	types	list	no?		global types to include
CompilerOptions	moduleSuffixes	list	yes
CompilerOptions	customConditions	list	no
CompilerOptions LS	plugins	list	mayBe
WatchOptions	excludeFiles	list	no
WatchOptions	excludeDirectories	list	no
TypeAcquisition	include	list	no
TypeAcquisition	exclude	list	no
Root	files	list	mayBe		#20110
Root	include	list	mayBe		#20110
Root	exclude	list	mayBe		#20110
Root	references	object	no		Transitive, vs listing only direct parent? #30608, #27098

📃 Motivating Example

A way to specify say project specific "paths" mapping without having to rewrite all the mappings from the base config.

💻 Use Cases

1. What do you want to use this for?
2. What shortcomings exist with current approaches?
3. What workarounds are you using in the meantime?