I’m curious as to why I would be getting a Typescript warning below.
There error appears on this line: a[k].toString().toLowerCase()
However I have the check if (a && a[k]) which should ensure that in the next line, a and a[k] cannot be undefined?
// Return coins that match text search by currency symbol or name.
export const findAsset = (txt: string, assets: IAsset[] | null[]) => {
// assets will exist here...
if (assets) {
// Typescript error here...
const checkText = (k: string, a: IAsset | null) => {
if (a && a[k]) {
return (textMatch(txt.toLowerCase(), a[k].toString().toLowerCase()) ? a : null);
}
}
const curriedCheckText = R.curry(checkText);
const byName = R.map(curriedCheckText('name'), assets);
const bySymbol = R.map(curriedCheckText('currency'), assets);
const matchNames = R.reject(R.isNil, byName);
const matchSymbols = R.reject(R.isNil, bySymbol);
const combinedSearch = (matchNames.concat(matchSymbols));
return R.uniq(combinedSearch);
}
else {
return [];
}
};
This is the shape of the IAsset interface
export interface IAsset {
[key: string]: string | number | undefined | boolean;
availableSupply?: string;
currency: string;
exchange: string;
exchange_base?: string;
marketCap: number;
name: string;
percentage?: number;
price: number;
position?: number;
value?: number;
inWatchlist?: boolean;
}
tsconfig
{
"compilerOptions": {
/* Basic Options */
"target": "es5" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */,
"module": "commonjs" /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */,
"lib": [
"dom",
"es2015",
"es2016",
"es2017",
], /* Specify library files to be included in the compilation. */,
"allowJs": true, /* Allow javascript files to be compiled. */
"checkJs": true, /* Report errors in .js files. */
"jsx": "react", /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
"declaration": true, /* Generates corresponding '.d.ts' file. */
"declarationMap": true, /* Generates a sourcemap for each corresponding '.d.ts' file. */
"sourceMap": true, /* Generates corresponding '.map' file. */
// "outFile": "./", /* Concatenate and emit output to single file. */
// "outDir": "./", /* Redirect output structure to the directory. */
// "rootDir": "./", /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
// "composite": true, /* Enable project compilation */
// "removeComments": true, /* Do not emit comments to output. */
"noEmit": true, /* Do not emit outputs. */
// "importHelpers": true, /* Import emit helpers from 'tslib'. */
"resolveJsonModule": true,
"downlevelIteration": true, /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
// "isolatedModules": true, /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
/* Strict Type-Checking Options */
"strict": true /* Enable all strict type-checking options. */,
"noImplicitAny": true /* Raise error on expressions and declarations with an implied 'any' type. */,
"strictNullChecks": true /* Enable strict null checks. */,
"strictFunctionTypes": true /* Enable strict checking of function types. */,
"strictBindCallApply": true /* Enable strict 'bind', 'call', and 'apply' methods on functions. */,
"strictPropertyInitialization": true /* Enable strict checking of property initialization in classes. */,
"noImplicitThis": true /* Raise error on 'this' expressions with an implied 'any' type. */,
"alwaysStrict": true /* Parse in strict mode and emit "use strict" for each source file. */,
/* Additional Checks */
// "noUnusedLocals": true, /* Report errors on unused locals. */
// "noUnusedParameters": true, /* Report errors on unused parameters. */
// "noImplicitReturns": true, /* Report error when not all code paths in function return a value. */
// "noFallthroughCasesInSwitch": true, /* Report errors for fallthrough cases in switch statement. */
/* Module Resolution Options */
// "moduleResolution": "node", /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
// "baseUrl": "./", /* Base directory to resolve non-absolute module names. */
// "paths": {}, /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
// "rootDirs": [], /* List of root folders whose combined content represents the structure of the project at runtime. */
// "typeRoots": [], /* List of folders to include type definitions from. */
// "types": [], /* Type declaration files to be included in compilation. */
// "allowSyntheticDefaultImports": true, /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
"esModuleInterop": true, /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
// "preserveSymlinks": true, /* Do not resolve the real path of symlinks. */
/* Source Map Options */
// "sourceRoot": "", /* Specify the location where debugger should locate TypeScript files instead of source locations. */
// "mapRoot": "", /* Specify the location where debugger should locate map files instead of generated locations. */
// "inlineSourceMap": true, /* Emit a single file with source maps instead of having a separate file. */
// "inlineSources": true, /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */
/* Experimental Options */
"experimentalDecorators": true, /* Enables experimental support for ES7 decorators. */
// "emitDecoratorMetadata": true, /* Enables experimental support for emitting type metadata for decorators. */
},
"include": [
"components/**/*",
"pages/**/*",
],
"exclude": [
"node_modules",
"styles",
"_document.tsx"
]
}
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Answer
There are two ways to that I can think of to get rid of the error.
The first way I can think of is to use a fallback with the || operator, which would turn this:
a[k].toString().toLowerCase()
Into this, so if the value is falsy, then use an empty string.
(a[k] || '').toString().toLowerCase() // Or with optional chaining a[k]?.toString().toLowerCase() || ''
Note: Use ?? to catch only undefined and null values instead of falsy values.
The other way is to save the value to a variable and check the new variable. Which then makes this
if (a && a[k]) {
return textMatch(txt.toLowerCase(), a[k].toString().toLowerCase()) ? a : null;
}
Become this:
let v = a ? a[k] : null
if (v) {
return textMatch(txt.toLowerCase(), v.toString().toLowerCase()) ? a : null;
}