Automatically ignore null keyword usage within getDerivedStateFromProps method

[SaeedZhiany]

Bug Report

• tslint-react version: 4.0.0
• tslint version: 5.17.0
• typescript version: 3.5.1
• Running TSLint via: CLI

TypeScript code being linted

public static getDerivedStateFromProps(nextProps: Props): Partial<State> | null {
    if(some conditions) {
        return {
            // some state values
        };
    } else {
        return null; // <--- I got tslint error here
    }
  }

with tslint.json configuration:

{
    "extends": ["tslint-react"],
    "rules": {
        "adjacent-overload-signatures": true,
        "align": [true, "parameters", "arguments", "statements"],
        "arrow-return-shorthand": true,
        "await-promise": true,
        "ban": false,
        "ban-comma-operator": true,
        "class-name": true,
        "comment-format": [true, "check-space"],
        "curly": true,
        "deprecation": true,
        "encoding": true,
        "eofline": true,
        "forin": true,
        "import-spacing": true,
        "indent": [true, "spaces"],
        "interface-name": [true, "never-prefix"],
        "interface-over-type-literal": true,
        "jsdoc-format": true,
        "jsx-no-lambda": false,
        "jsx-no-multiline-js": false,
        "label-position": true,
        "linebreak-style": [true, "LF"],
        "max-line-length": [true, 120],
        "member-access": true,
        "member-ordering": [
            true,
            "public-before-protected",
            "protected-before-private",
            "static-before-instance",
            "variables-before-functions"
        ],
        "new-parens": true,
        "no-any": true,
        "no-arg": true,
        "no-bitwise": true,
        "no-boolean-literal-compare": true,
        "no-conditional-assignment": true,
        "no-console": [true, "log", "error", "debug", "info", "time", "timeEnd", "trace"],
        "no-consecutive-blank-lines": true,
        "no-construct": true,
        "no-debugger": true,
        "no-duplicate-imports": true,
        "no-duplicate-super": true,
        "no-duplicate-switch-case": true,
        "no-duplicate-variable": true,
        "no-empty": true,
        "no-eval": true,
        "no-floating-promises": true,
        "no-for-in-array": true,
        "no-implicit-dependencies": [true, "dev"],
        "no-invalid-template-strings": true,
        "no-invalid-this": true,
        "no-irregular-whitespace": true,
        "no-mergeable-namespace": true,
        "no-misused-new": true,
        "no-null-keyword": true,
        "no-object-literal-type-assertion": true,
        "no-parameter-reassignment": true,
        "no-reference": true,
        "no-return-await": true,
        "no-shadowed-variable": true,
        "no-sparse-arrays": true,
        "no-string-literal": true,
        "no-switch-case-fall-through": true,
        "no-string-throw": true,
        "no-trailing-whitespace": true,
        "no-unnecessary-callback-wrapper": true,
        "no-unnecessary-class": true,
        "no-unnecessary-initializer": true,
        "no-unnecessary-qualifier": true,
        "no-unsafe-finally": true,
        "no-unused-expression": true,
        "no-var-keyword": true,
        "number-literal-format": true,
        "object-literal-key-quotes": [true, "as-needed"],
        "one-line": [true, "check-catch", "check-finally", "check-else", "check-open-brace", "check-whitespace"],
        "prefer-const": true,
        "prefer-for-of": true,
        "prefer-method-signature": true,
        "prefer-object-spread": true,
        "prefer-switch": [true, {"min-cases": 3}],
        "prefer-template": [true, "allow-single-concat"],
        "promise-function-async": true,
        "quotemark": [true, "single", "jsx-double"],
        "radix": true,
        "restrict-plus-operands": true,
        "return-undefined": true,
        "semicolon": [true, "always", "strict-bound-class-methods"],
        "space-before-function-paren": [true, "never"],
        "space-within-parens": false,
        "strict-type-predicates": true,
        "switch-default": true,
        "trailing-comma": [true, {"multiline": "always", "singleline": "never"}],
        "triple-equals": true,
        "typedef": [true, "parameter", "property-declaration"],
        "typedef-whitespace": [
            true,
            {
                "call-signature": "nospace",
                "index-signature": "nospace",
                "parameter": "nospace",
                "property-declaration": "nospace",
                "variable-declaration": "nospace"
            }
        ],
        "use-isnan": true,
        "variable-name": [true, "ban-keywords", "check-format", "allow-leading-underscore", "allow-pascal-case"],
        "whitespace": [
            true,
            "check-branch",
            "check-decl",
            "check-module",
            "check-operator",
            "check-separator",
            "check-rest-spread",
            "check-type",
            "check-typecast",
            "check-type-operator",
            "check-preblock"
        ]
    }
}

Actual behavior

I got an error with the message use undefined instead of null in return null statement of getDerivedStateFromProps.

Expected behavior

According to react documentation getDerivedStateFromProps must return null when there is no actual state change.

I completely understand we must use undefined instead of null in any elsewhere and I know I can ignore the error in that line, but I want to ask to add a feature that tslint-react ignore automatically such special case.

[Adi Dahiya]

Which rule is reporting that error? You can find out using the verbose or stylish formatter.

[SaeedZhiany]

I'm currently traveling right now and can't test. I provide all resources to reproduce the error, can you test it yourself, please?

Only a guess, it's probably produced by "no-null-keyword": true

[Adi Dahiya]

Ok, yeah, it's the no-null-keyword rule. I think this is part of a broader discussion of null restrictions here palantir/tslint#2798

This rule is fairly extreme and probably not suited for general use until we can sort out a good way to deal with the problems described in the linked issue. Closing as a duplicate for now