tslint complaining "statements must be filtered with an if statement" when using switch

Question

Lets say I have the following method:

getErrorMessage(state: any, thingName?: string) {
    const thing: string = state.path || thingName;
    const messages: string[] = [];
    if (state.errors) {
        for (const errorName in state.errors) {
            switch (errorName) {
                case 'required':
                    messages.push(`You must enter a ${thing}`);
                    break;
                case 'minlength':
                    messages.push(`A ${thing} must be at least ${state.errors['minlength'].requiredLength}characters`);
                    break;
                case 'pattern':
                    messages.push(`The ${thing} contains illegal characters`);
                    break;
                case 'validateCardNumberWithAlgo':
                    messages.push(`Card doesnt pass algo`);
                    break;
            }
        }
    }
    return messages;
}

when I run

ng lint

I get the following error :

for (... in ...) statements must be filtered with an if statement

Having a look at similar question I don't think that answer would be applicable to my situation. After all switch statement sits in the category of if-else-if ladder.

tslint should consider switch statement as form of if statement but it doesn't?!

Answer 1

This made me curious, so I checked out the TSlint source code for this rule. It has a function called isFiltered which seems to only check for ts.SyntaxKind.IfStatement, not for ts.SyntaxKind.SwitchStatement.

function isFiltered({statements}: ts.Block): boolean {
    switch (statements.length) {
        case 0: return true;
        case 1: return statements[0].kind === ts.SyntaxKind.IfStatement;
        default:
            return statements[0].kind === ts.SyntaxKind.IfStatement && nodeIsContinue((statements[0] as ts.IfStatement).thenStatement);
    }

}

So unless you want to convert your object to an array, you'll need to use a fix from the link you provided. Either Object.keys, or an if statement:

    for (const errorName in state.errors) {
      if (state.errors.hasOwnProperty(errorName)) {
        switch (errorName) {

The interesting thing is that you can have any kind of if statement and the error will go away. There is no check to see if you are calling hasOwnProperty.