Improve ts declarations (#27)

Co-authored-by: Valentin Semirulnik <v7rulnik@gmail.com>

Author: dragomirtitian

source-map.d.ts

@@ -35,12 +35,12 @@ export interface MappedPosition extends Position {
 }
 
 export interface MappingItem {
-    source: string;
+    source: string | null;
     generatedLine: number;
     generatedColumn: number;
-    originalLine: number;
-    originalColumn: number;
-    name: string;
+    originalLine: number | null;
+    originalColumn: number | null;
+    name: string | null;
 }
 
 export class SourceMapConsumer {
@@ -51,29 +51,35 @@ export class SourceMapConsumer {
     static LEAST_UPPER_BOUND: number;
 
     constructor(rawSourceMap: RawSourceMap);
+    readonly file: string | undefined | null;
+    readonly sourceRoot: string | undefined | null;
+    readonly sourcesContent: readonly string[] | null | undefined;
+    readonly sources: readonly string[]
+
     computeColumnSpans(): void;
     originalPositionFor(generatedPosition: FindPosition): MappedPosition;
     generatedPositionFor(originalPosition: SourceFindPosition): LineRange;
     allGeneratedPositionsFor(originalPosition: MappedPosition): Position[];
     hasContentsOfAllSources(): boolean;
-    sourceContentFor(source: string, returnNullOnMissing?: boolean): string;
+    sourceContentFor(source: string, returnNullOnMissing?: boolean): string | null;
     eachMapping(callback: (mapping: MappingItem) => void, context?: any, order?: number): void;
 }
 
 export interface Mapping {
     generated: Position;
-    original: Position;
-    source: string;
-    name?: string;
+    original?: Position | null;
+    source?: string | null;
+    name?: string | null;
 }
 
 export class SourceMapGenerator {
     constructor(startOfSourceMap?: StartOfSourceMap);
-    static fromSourceMap(sourceMapConsumer: SourceMapConsumer): SourceMapGenerator;
+    static fromSourceMap(sourceMapConsumer: SourceMapConsumer, startOfSourceMap?: StartOfSourceMap): SourceMapGenerator;
     addMapping(mapping: Mapping): void;
-    setSourceContent(sourceFile: string, sourceContent: string): void;
+    setSourceContent(sourceFile: string, sourceContent: string | null | undefined): void;
     applySourceMap(sourceMapConsumer: SourceMapConsumer, sourceFile?: string, sourceMapPath?: string): void;
     toString(): string;
+    toJSON(): RawSourceMap;
 }
 
 export interface CodeWithSourceMap {