File Download Helper Functions

helpers.ts

/**
 * Takes a HTTP response that represents file data and  parses out the file name from
 * the Content-Disposition and pulls the Blob object representing the body content of
 * the response.
 * @param response The response object that fetch returns
 */
export function getBlobAndFilenameFromResponse(response: Response): Promise<{filename: string, blob: Blob}> {
    // Get the file name
    let filename = 'unknown';
    let contentDispositionRaw = response.headers.get('Content-Disposition');
    if (contentDispositionRaw !== null) {
        let contentDisposition = parseContentDisposition(contentDispositionRaw);
        if (contentDisposition !== null && contentDisposition.filename) {
            filename = contentDisposition.filename;
        }
    }

    return response.blob().then(blob => {
        return { filename, blob };
    });
}

/**
 * Represents the contents of the Content-Disposition response header.
 */
export interface IContentDisposition {
    value: string;
    name?: string;
    filename?: string;
}

const keyWithQuotedValueRegex =  /^\s?([^=]+)\s?=\s?(([^"]+)|"([^"]+)"|\'([^\']+)\')\s?/;

/**
 * Parses the values of the Content-Disposition header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition
 * 
 * TODO: Support the filename* syntax.
 * 
 * @param contentDisposition The value of the Content-Disposition header
 */
export function parseContentDisposition(contentDisposition: string): IContentDisposition | null {
    let parts = contentDisposition.split(';').map(piece => {
        // Remove excess whitespace
        piece = piece.trim();

        return piece;
    });

    let firstPart = parts.shift();
    if (firstPart == null) {
        return null;
    }
    else {
        let response: IContentDisposition = {
            value: firstPart
        };

        parts.forEach(part => {
            let match = keyWithQuotedValueRegex.exec(part);

            if (match != null) {
                let key = match[1];
                let value = match[2];

                // The value might be a quoted string, so using JSON.parse()
                // will remove the quotes and properly unescape the insides.
                value = value.startsWith("'") || value.startsWith('"') ? JSON.parse(value).toString(): value;

                if (key === 'name') {
                    response.name = value;
                }
                else if (key === 'filename') {
                    response.filename = value;
                }
            }
        })

        return response;
    }
}